I just read the manual and it said magical benefits do not stack, so equipping a ring of protection and +1 armor do not give +2 AC. Does this also apply to weapons, namely ranged weapons like bows? Is there any benefit equipping + 1 or higher bows with +1 or other magic arrow types (arrows of ice for example)?

Bows are launchers, arrows are missiles. As to your question, it depends on which game you are talking about, since they are treated differently. In vanilla BG1 an arrow's THAC0 *and* damage are used. In BG2 only the THAC0 is calculated. With this in mind you need to be careful about equipping arrows with certain bows like the Gesen bow. The reason is that, although the THAC0 will improve, the damage will decrease, since it's no longer using its +4 magical arrows, but the ones you've equipped.

D&D2, where Full Plate armor is better than Full Plate +2 armor :))

This is classic Baldurs gate I'm playing.

And don't forget that in D&D 2nd ed you can use your full dex modifier for AC regardless of armor type. So a dex of 18 and full plate is a -3 AC before magic buffs

If you were answering to me, you may have misunderstood my point. To make my point easier, let's compare Full Plate (FP) and FP +1. FP is AC 1 and FP +1 is AC 0. On paper, FP +1 looks better. However, you won't be able to wear any protection cape or ring with it. However, with FP you can wear a ring of protection and not only get the same AC but also benefit from +1 Saving Throws. Rings are not really common on BG1 and IWD so you can't just use any other ring for your fighters.

I am about to finish IWD and for that same reason, I sold my Mithral FP +2 armors since I was not able to wear my amazing capes. My fighters are wearing simple FP and my cleric/mage is not even wearing armor yet still reached -12 AC. If my C/M wears that "amazing" Mithral FP +2 armor, her AC drops down to -8. Not to mention that her wizard spells get disabled. It is true that FP has additional protection (AC modifier) for slashing, piercing and missile but it just does not worth it.

The first part was to Hickory and to help make my original question more specific to the game I'm playing.

The second part was a nuance I noticed of 2nd ed vs 3rd ed (which I learned d&d on) were armor limits you max dex bonus allowed; so a high dexterity stat and heavy armor are often counter productive in 3rd ed. Mostly this observation apples to Neverwinter Nights and Temple of Elemental Evil which use this rule set vs Baldur's Gate and Icewind Dale which use 2nd ed.

Those differences can trip you up if you're not paying attention.

No, this does not apply to weapons.

For armor, there's a component in Tweaks Anthology that allows to wear multiple protection items.

Not sure if this helps at all, but there are stacking (sort of) bonuses with melee weapons when equipping something like the Gauntlets of Ogre Strength on a weaker-than-18/00 character and a +1/2/3 weapon. They aren't the same exact style or source of bonus, but the extra THAC0 and damage of the gloves (a result of the strength boost) piles on to the weapon bonus(es).

There is also a pair of bracers/gauntlets (weapon specialization something or other ?) you can get that provides bonuses to both THAC0 and damage, IIRC. And while the description reads melee weapons only, it seems to apply to missile weapons as well. The dude on the far eastern edge of one of the far eastern parts of the world map has them. Can't recall his name, but he's east of the Firewine Bridge area. I have also gotten an extra set of these when I first met and was attacked by Tazok. I ran circles around him, while I was hasted, drilling him from afar with arrows, before he could talk to me the 2nd time and end the encounter.