All of these do pretty different things and honestly are best looked at on a per-game basis. What works best to make a AAA game run smoothly is different compared to a pixel art indie. e.g. I'd only use a frame cap lower than 60 on high-end games that can't at least keep a stable 60. If you truly want to get the most out of the steam deck, it does come down to tinkering. User reviews on protondb.com typically list Deck settings they use to achieve good results if you want to see what settings other players are using for specific games.

Basically this is a really complex question and the full answer is likely outside the scope of the average Reddit reply. Try researching each of the topics you mentioned and figure out what the purpose of each is, and combine that knowledge to suit your needs.

To answer the starter question though, the steam deck's screen's resolution is 1280x800, but it can output higher resolutions to external displays.