In 2016, the Clintons ran a normal primary, but Bernie was creatively railroaded by the establishment and the superdelegates. The superdelegates went for Clinton 78% to Sanders 7%.
In 2020, COVID caused a lot of problems and Biden was basically selected without much of a real primary. Although he did secure the nomination and go on to win.
This year, due to party control they concealed the fact that Joe Biden was deteriorating. Fear of Trump was used to keep anyone from really challenging the incumbent. There was no real democratic primary here either. Due to campaign finance laws, the money donated to the Biden/Harris campaign is now locked to one of those two candidates. In effect, Kamala now is likely to be the candidate because of the lie about Joe Biden and collecting all the donations. For tons of reasons, the Democrats are now locked into a Harris campaign.
All of this is just a way to subvert a real democratic process...