How do you access, check, manage, use, add to Store Credit ?

Is this even a thing that exists ? Has anyone ever WITNESSED IT ?

No one knows right now...

This post in the FAQ should help you, and someone mentioned that a blue said Store Credit functionality would be happening soon.
Also take note of Question No. 6.

Wait so how do the 'Bonus Codes' that are going away differ from the GOG codes that are (occasionally) given away in bundles? the Groupees Be Mine 14 is still on and has a GOG Bloodrayne 2 code...
https://groupees.com/bm14

They differ?
You can have bonus codes that give you a single specific item, you can have bonus codes that apply a discount to specific items and you should be able to have bonus codes that add funds to the store credit. But they are all still bonus codes.

Well the FAQ isn't very clear then: point 1, line 1, 1st sentence:
"The store credit is replacing the bonus codes previously used on GOG.com. "

And it refers to a specific subset of bonus codes.

Yes, i put a regionally priced order through yesterday to check it out.

You can check out your balance on your shelf, it's at the end of the same bar where all the tabs are (movies, games, messages, etc), it's displayed as text on the bar, pretty low key but immediatley visible and in a good spot.

During the checkout process there's a checkbox with your current balance which allows you to either use the store credit you have or keep it if you untick the checkbox.

So let's say you have 4€ on your balance and you're making a 15 € purchase. You can either pay the full 15 and keep the store credit or you can use it in which case the order total will come down to 11.

It's very easy to figure out and everything seems to be working nicely.

I don't have it, so I assume that you have to make a purchase of a regionally priced product before the tab is activated.
Same was for the movies tab, until I "bought" the two free ones it wasn't accessible.

Yes, that's it. It's not a tab like the others though, it's just text displayed on the bar, regardless of which tab you are browsing (games, movies, message) your balance is always visible.