Yep, most buffs require concentration now. There are a couple exceptions but they're mostly the weaker buffs like longstrider. In tabletop 5e haste also doesn't let you cast two spells, , it's either an extra dash, disengage, hide, or a single weapon attack (no multiattack). Much much stronger in BG3.
FunctionFn
You don't really lower a target's AC, it usually stays constant. The AC is just the number you have to roll against to hit.
Ie, you roll a 10 on a d20. You have a bonus of +8 to hit. That means you'll hit the enemy as long as it's AC is 18 or lower.
Saving throws are the reverse. The enemy who's getting targeted by a spell gets to roll and add their save bonus to try and beat your spell DC, which is usually somewhere from 15-20 ish.
You don't really need to know the maths going on, it's all represented by the % chance that pops up when you go to target something. If a hit says 95% chance, that means you'll hit as long as you don't roll a 1 on a d20.
As for Karlach's AC, that's just a barbarian thing. Barbarians are usually unarmored, but they get to add their Con stat to their AC as well as their Dex. They have a lot more HP to make up for getting hit more often, and they resist all physical damage while raging so they're still quite tanky despite having a lower AC.
If you're on keyboard (I'm sure controller has it but idk the button) the T key will let you hover over keywords to get more information. Ie, if a spell says it paralyzed the enemy, hitting T will let you mouse over "paralyzed" and read how it prevents the enemies from acting, and makes all melee attacks against them crits.
I broke my own haste concentration at least twice a play session and I've been DMing 5e for 6 years at this point. Would have saved me a few wipes on tactician
This video shortly and succinctly shows this a few times: https://youtu.be/hSNWkRw53Jo?si=rCJA_3-QaANHSQX9
The most common form (at least where I'm from) of second person plural behind "you all (y'all)" is gendered: "you guys". It's used in an ungendered way increasingly commonly, but "guy" is still gendered to plenty of English speakers.
Controlling 4 digital 5e characters in rapid succession feels very different from controlling 1 character in a tabletop setting. Idk if it's better or worse this way, but (to me) they're pretty distinct experiences so it's worth at least trying
No problem, feel free to ask if there's anything else that isn't clear, it's a complex game