Experience, mainly.
I used to run a phpbb forum, on average the bot signups outnumbered the real people 10 or 20 times. And that was with some fairly robust anti spam measures in place - something I think this platform is too new to have properly sorted out yet.
I may be wrong, I don’t know how the back end here works, but any place where people can post publicly will be infested with bot signups very quickly. The only real variable is how good the anti spam measures are.
In my particular case (as was the case with most forums in the day), it was really just about spamming boards with links to whatever shitty ED pills or crypto scam they were trying to sell.
They were never really sophisticated, but never really had to be either. A spammer could spend a few minutes writing a script for a bot to crawl the web looking for phpbb signup pages, then try to create an account on any it could find and immediately post the links. They could post hundreds of links on dozens of different forums with just a few minutes work - and then do it all again tomorrow with a bunch of new signups.