It is unfortunately a common misunderstanding, and I must admit we didn't help the situation by leaning into calling servers "communities" on joinmastodon.org in the past (not anymore). There do exist plenty of "themed" Mastodon servers, aimed at specific communities, e.g. phpc.social which describes itself as a server for the PHP community, because some secondary discovery features of Mastodon lend themselves to what kind of feels like a slack/irc/subreddit, but ultimately, in their primary purpose, servers are service providers just like GMail/Outlook/Fastmail are for e-mail.