It’s all a little arbitrary. When you create a new service (like Lemmy, or Mastodon), you can have them link with anything, in any fashion you like. The defaults are mostly sensible.
For example, I’ve just made a mastodon post asking /r/casual a question. Once that synchronizes across, you’ll see the topic over there.
Well it worked last time. Truthsocial.com is still up.