1) pretty much daily, if not more. i mostly used telix for DOS, although I tried other things from time to time.

2) sometimes there would be ads in magazines, once on any given BBS, there would usually be text files available for download with listings of other BBSs and dail up numbers, usually by city / area code.

3) both. there were a small handful of dominant big name BBSs, usually with some limited free access and paid access beyond that with lots of dialin lines, lots of up to date stuff available for download, etc., basically run as a business like an ISP and with fulltime staff. Then there would be smaller, hobbyist BBSs with one or a few dialin lines, probably free or very cheap, but less stuff available for download, updated less often, or maybe just a part-time operation instead of 24 hours. various schools, clubs, magazines, etc. also operated their own niche BBSs for users too/members too.

4) mostly just like usenet group, mailing list, forum, etc. it's not that different from, say, reddit or stack overflow or something like that, other than being all text, shorter messages, and generally people would be posting using their real name / identity, and often discussions on BBSs would lead to meetups in person and vice versa, maybe you'd recommend your school friends to use or try a certain BBS. to me, that was the big difference vs the internet today where it is mostly anonymous and discussions never really lead to meetups or ongoing friendships.

5) a lot of the discussion was just about where to buy hardware, prices, buying/selling gear, and hardware / products themselves. a big part of it was just about distributing files too - software, shareware, images, adult content, etc.