What do you mean?

Any post, on any service, is technically accessible on any other instance, running any service. Actual implementation, varies.

Unless you run into it in the feed, the way to find a given post is to enter the original instance url for it into search on the instance from which you want to interact with it.

To upvote this post, for example, even from an instance that it hasn’t federated to, I can enter the url to this post on its host instance into search, and the other instance will fetch the post, allowing me to vote and/or comment.

Same goes for mastodon toots. Get the url, put into search, upvote, comment, whatever.