Ask HN: What outcomes of us election are possible in reality?

I see there is a lot of things going around about what can happen in the US election this year and I wanted to know what is really possible with the laws that we have today.

Outcome 1: Trump wins and stay in office.

Outcome 2: Biden wins; Trump vacates office gracefully

Outcome 3: Biden wins; Trump does not concede citing fraud etc and chooses not to leave office on Jan 20

Outcome 4: Election nullified.

I am curious if outcome 3 and 4 are legally possible? If so what would happen for each of these outcomes (3 and 4)? Like what would happen next and who would rule if there is an interim period?

Are there other real and probable outcomes possible here ? If so what are those? ( besides candidates not being able to compete in the election physically/mentally). Would be great to hear some perspective (preferably of people within the legal field)on what all could go wrong this year from a election perspective.