TIL that in the docs there is a detailed explanation for how federation and search works in Lemmy

https://lemmy.ml/pictrs/image/cb6a4782-4409-4f6b-9139-8c028785de1e.png

Guide: One way you can take advantage of federation is by opening a different instance, like ds9.lemmy.ml, and browsing it. If you see an interesting community, post or user that you want to interact with, just copy its URL and paste it into the search of your own instance. Your instance will connect to the other one (assuming the allowlist/blocklist allows it), and directly display the remote content to you, so that you can follow a community or comment on a post. Here are some examples of working searches:

- !main@lemmy.ml (Community)
- @nutomic@lemmy.ml (User)
- https://lemmy.ml/c/programming (Community)  
- https://lemmy.ml/u/nutomic (User)
- https://lemmy.ml/post/123 (Post)
- https://lemmy.ml/comment/321 (Comment)

You can see the list of linked instances by following the "Instances" link at the bottom of any Lemmy page.