I'm not sure how to answer this other than point to the SRD. There isn't much difference with 3 or 3.5 AFAIK, except you need to be proficient with the type of armour you wear or you'll get a penalty.
The typical armour adds to the AC, with a caveat on dexterity saving throws and arcane check.
Check pg. 57 for proficiencies, 62-64 for armour, 90 for combat
https://dnd.wizards.com/articles/features/systems-reference-document-srd
Other than armour, perhaps the different types of actions/bonus actions/reactions/interactions have changed since 3 and 3.5, to check. That's one area Larian went creative.
PS: the SRD document isn't the most friendly. It is *possible*, by looking - not on WotC of course, to find a PDF of the Player's Handbook
It's a nice book, by the way, the physical book too but it tends to lose its pages after a few times.