Traditionally, the party in power does not hesitate to endorse the incumbent for a second term (and third, in one isolated case that made perfect sense at the time). So far, all of them accepted except the few who failed to complete their first term (William Harrison, Richard Nixon, and John Kennedy). So chances are, if President Biden wants to run again, the Democratic Party would not endorse Hillary Clinton this time.
However, during his announcement that he will pick a woman for Vice President, Biden made it clear he is looking for someone who could replace him as the Acting President at any time. He sounded like a person who wants to be a one-term president. So if he chooses not to run again, his endorsement would be for a different woman: Kamala Harris.
Of course, I will vote for the Democrat who has the best chance of winning in November. in 2024. If Biden is that person, I will vote for him. If Hillary Clinton runs against Harris, I will vote for Clinton because she has all the qualifications and won about 3 million more votes than Donald Trump in 2016.