Fun fact! Your local instance creates a shadow account for remote users, which has a unique ID and a copy of the profile data, but is otherwise empty.

I haven't seen this documented anywhere and it cost me about a day of effort when I was building a mastodon bot. If you naively try to fetch content from a remote user replying to a post, you won't find anything. You have to hit the remote instance's search API and pass their profile name in. Which is frankly insane.