Many times the Biden campaign doesn’t announce the time etc of rallies because more Trump supporters than Biden supporters show up. I think the polls are so far off the mark as to be ridiculous!
You’re correct, that they barely announce them; I’m kinda wondering if to get attendees, the campaign calls up a local Dem with a business and says ‘hey, we’ll give you X amount of $ if you’ll send over 30 employees for an hour’. Or they go to some union shop or union training facility where they have a captive audience.