Can I use assets made by other people (sprites and stuff), or is it only assets made by myself?

AFAIK you can if they are royalty free assets that are obtainable by anyone. I can't speak about paid assets or unclearly sourced assets.

I don't see why not, providing that you have the appropriate licensing. I'll talk to Apos before confirming first.

I'm not considering bought assets, however since I intend to use this jam as a reason to familiarize myself with Monogame and I'm not to good with art. I want my focus to be in programming and not art. My intention is to use free assets from itch and similar sites if allowed

You are allowed to use assets made by yourself or others. The assets can be created before the jam.