For example in the screenshot (twitch clip), if you want to share on reddit you click the reddit button, but what button are devs supposed to put to share in the fediverse? Do they pick an instance of their choice?! Is this a weakness of a federated platform or what?
Ultimately, some user account has to create a post to share something. Each account is tied to an instance, so: Yes.
Through federation, the post is visible to other users on other instances.