In the simplest terms, each state holds primary elections to decide who should be a particular party's candidate for the presidency in the main election. After those primaries, each party holds a convention to formalise the nomination. Each state has a number of electors depending on population size and a candidate gets the vote of all of the electors from that state if they won that states primary. At the convention, each elector casts their votes in accordance with their states result and the nomination is formalised. The convention is a big event with speeches, rallies etc.
The primary elections when there's an incumbent president running for a second term are usually a formality because other people in the party rarely run against a sitting president.
Because the primaries have already happened and the electors are already bound to Biden, they can't easily replace them unless he choses to stand down.
If he choses to stand down, the electors are no longer bound by the primaries and can cast voted for any suitable candidates. The chances are, if he did stand down, the party would try and unite behind a candidate to avoid a messy fight at the convention which wouldn't look good on TV.
It's so different from our system where we vote for a party in our area and the party with the most seats appoints a Prime Minister who is essentially unelected but holds less power because they are accountable to parliament and their party. In America, the position of president is more powerful with less accountability and the process for electing them is very specific and separate form electing their representatives.