The DNC nomination process is fairly democratic, but not entirely. Ultimately, the party delegates who are sent to the convention decide who the nominee is. By the time the convention happens this is usually a formality, but there have been open or brokered conventions before. You can look up and read very quickly how the process works, it’s not that complicated.
The reason people were mad about 2016 is that the DNC has rules that allow party leaders to put a thumb on the scale (sometimes a heavy thumb or multiple digits) if they don’t approve of how things are going. This is why Bernie wasn’t butthurt about losing whereas his following was; he knew the rules going into it, that not only would he have to overcome the superdelegates, but also anything else the party luminaries had in their back pocket along the way.
All that being said, the delegates who go to the convention are determined by the state primaries and are all for Biden this election. This means that if Biden does drop out, they are likely going to tow whatever line the higher-ups in the party want, which will be Harris. In the unlikely event that they don’t, there could be more of a contest, but they still have to reckon with the superdelegates.