Trump is definitely going to be the GOP frontrunner if he choses to run again tonight. He is still popular there, is a debate monster and the MAGA cult is still going strong. As Dave Chappelle said, never underestimate the poor whites.
However, I do not think that he will do a Grover Cleveland and win. The element of surprise is gone. The media, as someone else has already said, is not backing him this time and the midterms are already a telltale indicator of what is going to happen next. A Trump nomination is basically a guaranteed four more years for the Democrats.
As for Biden, if he is healthy enough, he will run. It is very difficult to say no to power once you have it, especially if there is a chance to have it again for the same amount of time. As far as I know, LBJ is the last president to say no to reelection (he finished JFK's term, won his own in a landslide and could have run for another one but chose not to). I am not sure if it was his failing health or the unpopularity of his administration that made Johnson decline to run again (unpopularity AT THE TIME, since now LBJ is widely considered a top tier president), but unless Biden has equally strong reasons not to run, nobody in the Democratic Party is going to stop him.