Posts can also get synced if someone from gram.social searched for a direct link to a post. E.g. if you go to gram.social and search for https://pixelfed.social/p/pixuser/1234, it’ll get synced in the background and show up if you follow @pixuser later on. (Depending on how long the instance caches it.)

Likes and Reshares of external posts are local to their instance. Those won’t get sent to other instances. E.g. on my private GotoSocial I don’t see the correct amount of likes or reshares of posts from other instances. I’ll have to visit the posts on their original instance for that. (But then, I rarely care.)