How can I link a post such that everybody would open it via their home instance (and therefore could interact with it)? Or is there at least any way how to open specific post (from another instance via my instance) manually?
Not supported yet, last I checked (within the last two weeks). There's a feature request open on the Github.
I've mainly just been navigating to it manually by opening the community, it's usually a post near the top when I've had cause to do it. You can also use the search functionality.
https://github.com/LemmyNet/lemmy-ui/issues/1048
In a nutshell, many people are bringing in more or less useful comments although it has already been suggested how this could work; the changes required would touch some core functionality in the way federation is done/ links are handled, thus none of the new devs take the initiative; the two main devs are occupied with other serious stuff.
Can't be done (outside of apps) without a browser extension. I can imagine a feature like that getting baked into some browsers in the future but for now you need to set it up yourself.