For the 2nd, based on my experience you are likely to find developers from both big and small companies equally hanging out in communities. Most lean technical btw, so if you arent targeting developers, communities probably arent the best venue to find potential users.

I also forgot to add: finding followers of OpenAI in Linkedin might also be a good way to find ChatGpT customers. Or people who comment on their product announcemnts in Linkedin.