This is an area of very much needed improvement.
For users, you should use the completion popup. When you type an @ followed by a name it will suggest users matching that, and if you click on one it will fill out a mention with the link to their origin page.
Give it a shot, try mentioning me in a reply!
The issue with this is the link isn't portable, meaning it may get you out of your home instance. The best way to link any user or community is to do /u/name@origininstance.tld
(for users; /c/
for communities) instead of https://origininstance.tld/u/name
. That way the link is relative (so no issue of moving away from your home) and it's fully qualified (so no issue with name clashes between instances).