Well, I’m not sure how it will work specifically in BG 3, but in D&D 5E (which BG3 is based on, but does change some rules) your AC is generally determined by your type of armor (wearing no armor is 10 AC, and then any armor will be higher than that based on the quality of it) and your Dexterity modifier. Now some medium and all heavier armors will restrict the Dex modifier bonus, some limiting it to +2 (you can still have your Dex stat higher, you just won’t get the bonus modifier for your AC more than +2) and the heavy armors usually ignoring Dex entirely (usually these are about 16-18 AC).
You also have Shields which add +2 though you need proficiency with them as well.
Like in my first D&D campaign, I had a Dwarf Rogue who had light armor proficiency and her armor was 11. At level 1, her Dex was 15 so she got a +2 modifier so her total AC was 13. When she hit level 4, rather than increase a stat by 2 (or two stats by 1 each), I took a Feat that gave her +1 Dex and proficiency in medium armor and shields. Her Dex was now at 16 so she has a +3 modifier now plus she bought a shield so an extra +2, resulting in an AC of 16.
If Feats are in the game, you should be able to get one at certain levels that lets you be proficient in medium armor and shields (and I see as I was typing this someone confirmed Feats are in the game). Mind you, this does mean you miss out on adding 2 to one stat or 1 to two different stats, so think carefully before going for a Feat (the medium armor one does give +1 to Dex so it does have that at least).