How would that work? The post exists on that instance. You're home instance just caches a copy. To share from your home instance the post would need to be recreated on that instance, so a new community with the same name would need to be created with it.
this post was submitted on 06 Sep 2023
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From the Lemmy.ca website, when I share, for example, the Reuters post on Toyota, you can see that the link is a Lemmy.ca link, even though it's federated from lemmy.world.
Yes, you are viewing the post that was made on Lemmy.ca, federation allows you to see posts on other instances. You are able to see the post and comment because of federation, lemmy.world grabs a cached copy to show it's users, the actual post is still on the original server. If you want to share a post, and let others interact in the comments, the link needs to be to the actual post. If you just want them to see something, and not know where it came from, a screenshot might be better.
You can interact with the post if you're signed up to Lemmy.ca, which the users I'm sharing the posts with are.
I can't interact with the post from lemmy.world's site if I don't have a lemmy.world account.
And realistically, you would only want to share content through your own instance, as it's the only instance you'd want to vouch for and can guarantee it is free of advertising, lewd UI themes (ahem, yiffit), bad redirects, or anything that could weird out someone visiting the content you're sharing with them. There's also the "you always send me links from random websites" factor, and people are rightfully hesitant to click on links from domains they've never seen before. If they know person X always shares posts federated to their own instance, they'll be more likely to trust the link.
To share from your home instance the post would need to be recreated on that instance, so a new community with the same name would need to be created with it.
That is essentially what already happens, when you subscribe to a community from another instance, it creates a community on your instance with the same metadata (sidebar contents, icon, etc) and appends the @instance.tld suffix to the end of it. Your instance then sends a message to the original instance saying "Add me to your mailing list for posts/comments/votes/moderator actions for this community". When you're viewing comments and posts from a remote instance, you're viewing the copy of it that your instance has (hence why post and comment IDs are not global - they are unique to your instance), its not fetched in real time.
After one of those new actions gets added to the original community's instance, it replicates that action to all instances that have at least one subscriber to the community. It also works in reverse, when you cast a vote, reply, etc your instance sends that action to the original one so that all (subscribed) instances have a copy of it.
If I were to say, block all IPs on my server except for my own IP, I'd still be able to view this post because my instance's database has a copy of it, I just wouldn't receive anything new on it. This has been noted as a potential issue for deleting data and following various regulations, because if you were to delete this comment after I took my server offline (or changed Lemmy's code to not honor deletions), I'd still be able to see your comment because my instance never got the signal that you deleted it - there is no way to guarantee deletion of content on the Fediverse for this reason.
This is also why every now and then you get "vote drift" where the votes on comments and posts aren't in sync, because the signal that a new vote was casted didn't make it to your instance for some reason. Or if someone comments from an instance that yours is defederated from (but the original hosting instance isn't defederated from) your instance won't show those comments, but if you go to the original instance (or even the one from the instance that was defederated) it will show up.