There are instance-independent links to communities, for example: [email protected].
The good thing about it is, it will keep you on your home instance logged in while visiting the target.
How to make a similar thing to link to specific posts, or specific comments?
I cannot see the comment on my instance, but still wanted to reply to it
This is true for now, but some good news is that work is done to change this, which you can see on Github