He was removed from the ballots in a couple states that he wasn’t going to win in anyway. In the most democratic country on earth, winning states is the only thing that matters, not total votes received. For any given election, only 5-10 are really contested - the others are essentially conceded by the candidate expected to lose there. So that’s why you only ever hear about poll results from places like Arizona and Michigan and not California or Alabama.

He is, the Republicans are behind him basically 100%, they’re officially nominated in June (August if their party is already in charge), he’s been removed from like two ballots so far with some other petitions in the works (but they’re all blocked until the supreme court can rule on it), and most* republicans won’t care if he gets arrested.

*Keyword being most. If he DOES get arrested, enough republicans would likely stay at home for Biden to eke out a narrow lead. Not many republicans, though.

He is running.

As far as I know he's not on the Colorado ballot, which is largely inconsequential to the overall race.

Yes he's having a million charges, but those funny disqualify him from running. The only charge that would is treason, which he hasn't been charged with, but that's being used to to keep Trump off some states Ballots.

Candidates are formally nominated at each party's convention in July and August, but usually who they nominate is known in March/April.

Republicans were trying to get other candidates to run last year, but no single candidate has yet proven to be as popular with Republican voters as Trump has. Not even close. The Republican race is now between two candidates, Donald Trump and Niki Haley. Trump is still far and away the leader.

Remember when thinking about US political systems as a non-US person, the US is much more similar to the EU than it is to any single member within the EU. Each US State has its own unique primary election process where they elect representatives to go to the national convention to choose candidates.