As I recall, the Bidens had nothing to do with the selection of Harris as the nominee. They were out of the decision making loop.
I think Pelosi had something to do with picking Harris. And maybe a few other bigwigs. But not Biden.
Kamala Harris, the 49th vice president of the United States, announced her 2024 campaign for president on July 21, 2024. On that date, incumbent president Joe Biden withdrew his re-election campaign and immediately endorsed her to replace him in his place as the party's presidential nominee.
I fully understand that there are mechanics involved. States, and committees, influential party figures. Nothing is automatic, and people engage in maneuvering to make any nomination official. I understand that Biden could not "wave a magic wand" and just make Harris the nominee. But I think it is true that Biden basically said, "I'm dropping out and I endorse Kamala Harris as my repalcement" and that complicated everything for the Democrats.