Using @mentions to search users’ posts has been available for a long time. I believe the tip in Search tips and tricks 🔎 is to find when people have @mentioned someone within a post itself rather than posts by them.
That’s right. 
The search will look for posts with all the search terms in. Currently there is no OR function for it.
For the regular search, it works on stemming, though if you’re using the AI semantic search it will find more topics based on a similar function to the Related Topics you find at the bottom of topics.