No I think they just show up randomly. You’ll have to look out for Lemmy posts to see if anyone from Mastodon comments. Next to their name is their instance, so if I were on Mastodon, my username would be something like: Resonosity@mastodon.social or something
This is one of the reasons why the US federal government wants to ban TikTok, a highly unpopular among the US general public. TikTok isn’t moderated in ways that suit US ideologies and propaganda, which means more leftist content leaks through to the masses.