Armor and Magic Resist on non tank characters

totally depends. Are you safe? Just build damage if so. Do they have assassins that can reliably reach you? Build a defensive but dangerous adc.

Use armor per level and magic resist per level, with your damage/lifesteal you can gain alot of levels

I always think of the 5 item master yi jungle in this scenario. Against a bad team, he builds full damage and shreds them one by one. Against a good team, get gets chain hard cc'd and deleted in 2 sec. Which is why when there are fed burst enemies around and enemies with cc, I always ask them to build at least 1 or 2 defensive items. Sure you lose some dps, but if you would die halfway through a teamfight due to going full ad, building 2 tank or resistance plus damage items might just see you through the full fight and let you end up doing more damage overall.

If you use shields/lifesteal, resistances make them stronger

If AD squishy vs AP squishy = If AD squishy vs AD squishy = or

If AP squishy vs AD squishy = LOL pray to our Lord and Savior (jk or ) If AP squishy vs AP squishy = or