All the delegates have been selected for the Democratic National Convention and they will attend. Prior to Biden stepping down as a candidate, they were required to vote for Biden in the first round of voting (which would have given Biden the nomination). Now, they are "free agents" -- since Biden is not a candidate anymore, they are free to vote for whatever candidate they like.
So, there will be nominations for President at the convention and the delegates will vote for their choice of candidates nominated. This will then continue until a single candidate gets a majority of the vote. This may happen smoothly, where a majority agree quickly to vote for a single candidate, or it may be something that lasts for several rounds of voting. If there are several rounds, there likely will be back room deals where lesser candidates are convinced to drop out, telling their supporters to vote for another candidate, in exchange for the candidate dropping out to be guaranteed an office in the new administration, such as becoming the VP candidate.