Sometimes, but not always. Lemmy is forum software and each website that hosts it is an “instance” of the lemmy software. Some are general focus such as lemmy.world and sh.itjust.works. Others are more themed like lemdro.id and lemmy.studio

Ones with a a focus are just more likely to have communities (subreddits) based around that theme but you can still subscribe to [email protected] from your account for example.