“When do they actually pick their nominee?’
The democrat 2020 presidential convention will be on July 13, 2020. Some think the eventual nominee won’t be picked until then, but I expect by then a candidate (Joe Biden) will have accumulated enough delegates so it will essentially be a coronation.
Or, after 48 months of campaigning against Trump, they do NOT solidify around one candidate (Biden ) , and so Hillary or Ophra or Michelle is “coronated” (from the floor) on the second ballot.