Tags vs. Pages
After a lot of experimenting, I’ve settled the issue like this:
- Tags are meta-information about the content, they tell me what kind of note it is, or what its status is, or what context it belongs to, not what it’s about.
#article,#recipe,#person,#book,#meeting, etc. - [[pages]] are for entities, or subjects.
[[Digital Physics]],[[New Orleans Shrimp]],[[Ted Nelson]],[[A Clockwork Orange]],[[January Board Meeting]], etc.
Using your example, topic XYZ would probably deserve its own page [[XYZ]], as well as [[PersonA]] and [[PersonB]]. Inside [[XYZ]] you could collect information about that topic and add wikilinks to related notes; on the other hand, the notes [[PersonA]] and [[PersonB]] could be tagged with #person.
Backlinks
Backlinks tell which pages link to the one you’re viewing. So, if you create a link to [[Meeting with John]] in your daily note 2021-01-09, when you visit that page you’ll see 2021-01-09 in the Backlinks pane.
Hope this helps.