Put simply, yeah. lemm.ee can communicate with lemmy.world, and kbin.social, and mastodon.social, as they're all "federated" with each other (like a federation of nation-states).
In the future you might see bsky.social and bsky.io and bsky.jp etc etc, and they'll be able to talk amongst themselves, but they won't be able to talk Lemmy or kbin or Mastodon.