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.