Most of them are actually paid to be there. There are ads on craigslist for rent-a-mobs. I don't know exactly who IS paying them, but you will notice that their signs look to be professionally made too. And how about the convenient pallets of bricks that seem to be just where they need them?
If you look at the latest No Kings protests, most of the protesters are people in their 60s or older trying to relive their glory days as Vietnam era Anti-War protesters, which coincidentally, is right on topic here.
Mods, not meaning to Go There, but if I once again have, feel free to delete my comment.