Notion syncs using https. It’s safe to say that as long as you haven’t specifically installed weird apps (notion is not a weird app) nothing going on on your PC is visible to anyone else.
This is of course, not true of enterprise and school devices, which usually have very powerful antivirus solutions installed that allow the work/school to see whatever you do (though they mostly don’t care, as long as you aren’t causing trouble on the network or doing things that might get them sued)