BG3 might have multi-classing, but with a level cap of 10, it will be really questionable if it's worth bothering with for anything except Warlock 2 - 8 Something Else. If you take a third level in any class, then - rules as written - you will miss out on the level 8 Feat/Ability Score Increase. Most classes get their cool archtype with features at level 3.
Ability score is based on player level, not character class level iirc.
