Does Plex keep track/logs of content that is shared between users? No
Does it keep track of what content is present in our libraries while connected to myplex? No
How come I receive emails what my family user is seeing?
Besides, the events recorded in the Activity feed are not necessarily indicating that a user has watched a certain title in his own (or another user’s) personal Plex server.
It could equally just been marked as “watched” on watch.plex.tv or streamed for free from https://watch.plex.tv/de/movies-and-shows