Ok thanks for the clarification.

From these discussions, I sometimes get the feeling as if elsewhere there are only two options to buy a desktop (gaming) PC:

1. Buy a prebuilt supermarket PC (HP, Dell or whatever) which is possibly overpriced and has a non-optimal configuration and poor expansion capabilities.

2. Buy all components one by one from dozen different stores, and then assemble them yourself, starting from applying thermal paste to your CPU etc.

But maybe I've just misunderstood something, and those "pre-builts" similar to Jimm's or Verkkokauppa.com are similarly available in other countries too? All the desktop PCs I've ever bought were those kinds of "pre-builts" where some (usually small) PC store has built the system from the components that I've chosen. I've never installed a CPU, used thermal paste or installed a motherboard to a PC case, ever.

And never will. I get about as much enjoyment from assembling a PC myself, as I do from assembling a piece of IKEA furniture. Ie. none, I hate doing it.

Ok so I am still confused. So pre-builts ARE referring to those "built by the store from stock parts" PCs, not brand-name desktop PCs like HP or Dell.

So I still don't understand, what is wrong with such pre-builts? At least back when I have bought them, the PC store has given me free hand to alternate the components, e.g. if I wanted more RAM or different kind of RAM, or a bigger hard drive, or a beefier graphics card. They just replace the basic configuration with the components I want, build it, and I can get it assured that the components work together as they have tested that.

This is a good example:

https://www.xmg.gg/en/desktop-pcs/xmg-trinity/

So they give a few base options for a desktop gaming PC (either Intel or AMD CPU based), and then you have several options to configure what kind of RAM, what exact CPU, what graphics card, etc. Is this still considered as a pre-built?

Is there something wrong in buying a pre-built like that, instead of buying all the components separately and then trying to assemble them yourself, like assembling a piece of IKEA furniture?

There are several possibilities of pre-built PCs here.

1. Large electronic stores
2. Smaller specialized PC shops
3. Supermarkets from time to time
4. Large online stores/websites which sell everywhere (Amazon, Ebay, etc as well as on their own website obviously).

Of course, on those large websites, you can also let them build your custom PC, you can order individual components etc.

There definitely was a very large variety here last time I bought a PC (when everyone and their dog still bought those, before it shifted somewhat towards smartphone/tablet crap). So it might not be as varied anymore but considering there is still a lot of export happening, it should be fine.

My one is third from the right on choose a case, thermaltake. 677mm high, smaller than I thought. Still way bigger than needed. Mind you the 3080 inside is the size of a small laptop on its own!
https://www.pcspecialist.co.uk/computers/intel-z590-pc/
Any computer should run all old games. I tend to avoid onboard graphics as there seems to be issues with them. With FPS it’s all about tweaking settings until you get something good. For instance you can get GTA5 running on a gpd win 2 which is a tiny handheld, but it does require tweaking. Special effects,post processing, detail etc. All contributes.
HD (the 1920x1280) is standard size now, a lot of people are pushing 3 or 4K, a good monitor can help, especially gsync if you go nvidia, or free sync if other. Ssd and loads of ram is a good idea.

Nothing's wrong with anything, it all depends on your expectations/requirements/budget.
That XMG in your example is something between "fixed configuration pre-built" and "choose every and any component yourself". It's pretty great if you don't want to do all the research.

I started reading up on it in the late 90s, because I wanted to get the absolute maximum out of my small budget (and also just because the subject interested me). I know my expectations/requirements/budget well and can pick out the best components that match it, a lot better than any other person/store can do for me.
That doesn't mean that I have to assemble it myself. These days, I let a trusted store do it for me - mainly because I don't want to pay extra or have the extra fuss if something goes wrong - however rare that is.

Back to your example...
Let's look at the CPU coolers available for that model. The only have one (air cooler) selectable, though that Dark Rock is a pretty good one. But me, I might want a certain Noctua model instead, because it's quieter. You on the other hand, would rather not spend hours/days online reading reviews and comparing specifications... (and that's fine).

When you buy a fixed configuration pre-built, you let the store decide the (component) priorities to keep a model within a given price range. Also OK.... if you're fine with their choices.

That PCSpecialist seem very expensive. One system linked to earlier from XMG was about 2100 euro - probably not inc UK VAT. A similar spec system at PCS was coming in a £3400 inc VAT! I don't need to spend that much on something that high spec, most of it's power would go unused with what I'm likely to playing. Although I say *likely*, not definately!

Oh, I don’t recommend them anymore. I got my previous one from them and that was a good experience, had an issue with the sound card and they sent a new one to swap over in a couple of days. So was pleased. Got my current one though, took months - that’s not their fault, major chip shortage - but the communication was terrible, and I paid for a full professional copy of windows, only to find out it was built into the motherboard. They were less than helpful on that. So no, I don’t recommend them, and don’t think it’s great for the additional If you have the time, and willing to risk breaking something, then build yourself, it’s cheapest. Prebuilt are mid range, some are ok, the days of memory glued into the motherboard are long gone thankfully. Custom is just that, high end build as you want.

"similar spec" is a bit vague. A different GPU can make a HUGE price difference... because GPU prices are currently over-inflated.
It's just a bad time right now to buy a GPU, especially if you want to run modern games at high gfx settings / resolution / framerate.
I'm not upgrading until those prices settle down. I just play 'older' games and everything else goes to my buying/playing backlog. In a way, I've always done that. I have a budget, a maximum amount of money that I can / I'm willing to spend on a graphics card. When a certain game is more demanding than that - I just backlog it and play it after my next upgrade.
We're living in digital distribution times - why the rush to play the newest of the newest? (Social pressure? Boohoo! Deal with it!)

That's the main reason why I personally would go with a "pre-built", meaning something like those that XMG offers, ie. I can fine-tune which components go in (e.g. what kind of rating the RAM has and that it is used dual-channel, and whether I get a RTX 3060 or 3070 or whatever).

First thing is that I hate the idea of assembling it myself. It is just extra work like assembling and IKEA piece of furniture or trying to fix my car's oil and brakes, and I don't really get any enjoyment out of it. I get the feeling some people actually enjoy that kind of tinkering, but I don't, really. The most hardcore thing I've done to a PC is replacing a broken PSU with a new one, and replacing a broken fan within this laptop I am writing this. I did it because I had to, not because I enjoy it or anything.

I've never attached or detached a motherboard from a PC case, put a CPU into its socket, changed the thermal paste to a fresh one etc., and I hope I never have to.

The second and probably the more important one is that I want the store to make sure the components work together. I don't want to end up in a situation that some components wouldn't work together for an unknown reason, and then I'd have to inspect myself where the problem lies, and then start arguing with the manufacturers or suppliers whose fault it is they are not working together. Nope, I want the store to guarantee that the components they've put into the PC work together fine.

I guess there is also the thing what you say about having to research different components etc. I wish the store has done that research already, and tells me what advantages and disadvantages different choices might have, and why they've chosen certain parts.

Just for info.......
https://www.ebay.co.uk/itm/174865288263
That's the Acer Predator G3-605 I had. Still have actually (awaiting burial).
More expensive than I expected, there's two on EBay here, £300. One was £350 but I can't find it again! I'd have estimated £200.
Tempting though it is, I have never bought second hand or refurbished PCs. And I would never ever touch EBay (or any other similar retailers) with a 10 foot shitty one.
On closer inspection, these would not even be exact replacements. Their spec stickers are different from mine! Inferior looking too.
Mine had an i7-4770 quad core CPU, these have i5-4460s, I may be wrong but that sounds like a lesser CPU model.
Mine has 16Gb DDR3, these have 8Gb DRR3. Nah, 8Gb just ain't enough these days IMO. Mine had a 4Gb GTX 745. One on Ebay has just a 1.5Gb 760 in it's sticker (yet claims 4Gb in it's "store title" - dodgy!).
These have 1Tb HDs, mine had a 3Tb.
Both say Windows 8.1-64 on the sticker, but in the title say Windows 10. So W10 has been manually installed over W8.1 then. I did this with mine in 2016 when the upgrade was still "on a free offer". Rolled it back after 4 days. I hated it.

PS/ I have to agree with Timppu, as I also have Z-E-R-O hardware skill. Major stores peddling stock PCs and PC builder services like PCSpecialist really have me by the balls quite frankly. And that's exactly how they like it.

As for playing the most modern games, or feeling beholden to do so, well I have no so social pressure issues. In fact the lastest games over the past few years may have been technically superior, but they seem much less "interesting" to me. Also there is the current state of the game industry. Developers big and small treat us like shit these days. Releasing broken bug riddled unfinished crap, prices going up and up. For that you get overpriced MINIMAL content games, with pre-meditated "long term plans" to fill them out with endless overpriced low content DLCs over the next decade. Customer service and respect for customers is utter shit. Companies in other industries would go bust with such attitudes. DRMs become more and more draconian, insinuous and convoluted. GOG thankfully still disallow such crap - but these developers just wont release on GOG, they stick to Steam etc. That's why I haven't partaken in this circus for the past few years.