That frees the delegates to vote as they please. Anything can happen from there. They'll still be loyalists, so they would be likely to start from Biden's wishes on a successor. With him out of the game, though, they would no longer be beholden to him. They would also be looking for a new patron, so they would listen to offers from insurgents.
Biden doesn't control the selection of his replacement. All he can do is make his wishes known.
I would be very surprised if Harris walked out with the nomination. Her power base within the party is narrow. Newsom is in a much better position to deliver on commitments in California, and she is short on natural allies at the national level.