How does it work for him to step down, who's decision would it be and does he have to agree?
That is one deep rabbit hole.
To replace Biden at the convention by the rules, one would have to persuade 2000 Biden loyalists to vote against him. If they don't line up behind the same person, then the 700 or so superdelegates join the party, and they vote until they settle on someone. This is not considered a likely outcome.
The procedural way to get Biden out, if all else fails, is the 25th Amendment. That absolutely requires Harris and eight cabinet secretaries. As I understand it, "Cabinet-level officials" such as the director of the OMB, CIA and Ambassador to the UN don't count.
Any eight will do. However, two of them (Labor and HUD) have acting status, meaning they have not been confirmed by the Senate in that role. In principle this means seven would be enough, but Biden's team would immediately file for relief in DC federal court and all hell would break loose. Ditto if they get to eight with an acting in the majority. In practice, this means a supermajority of eight of thirteen.
Blinken is a hard "no". It would be a very difficult sell to Garland. Buttigieg has cause to avoid sullying his hands with it. It wouldn't be a good look for Austin after his December fiasco. Yellen might be gettable.
That leaves it in the hands of the wild cards, and your guess is as good as mine what they would do. If the Cabinet votes are secured, then a third of the Democrats in both houses (plus one in the Senate) would have to join the Republicans,
assuming they vote in favor, once Biden's team says he's fine and forces the vote required by Section 4 of the 25th Amendment.
TL;DR: It's all but impossible unless Biden falls on his sword.