BBS lists were published in computer magazines and on other BBS systems in the same area code, generally. Once you found one you quickly found links to others in your area.

Some guy in your city would just leave his computer on all day and you could call it over a regular phone line with your modem. Only one person at a time could connect and if someone else was on that board you just got a busy signal.

Terminal software and later modems themselves had “autodial” features that would keep trying to call until they eventually connected, so if you wanted to call a specific board you’d just wait while your computer dialed and hung up and dialed and hung up over and over again until it heard a modem on the other end. It was a huge technical innovation when US Robotics invented a modem that could detect the busy signal, allowing it to try the next attempt much sooner. Earlier modems just waited 30 seconds for either a connection or nothing and timed out before trying again.

In the late 80s BBS software started supporting interconnections where you could call your local BBS and send an email to a user on a completely different BBS, even in a different city. This could take multiple days to send and then more days again for any potential reply. It felt like Star Trek at the time.