What can employers see on personal devices?

What all are IT admins able to see about employees via wifi connected personal devices?

For example, I know on a work device pretty much everything can be seen such as search history and Microsoft Teams messages, etc.

What about someone who connects a personal phone to the wifi? What information can be obtained? Especially if a person isn’t very conscious of privacy?

I’m a newbie to privacy but for context, a friend of mine had a “scandal” at work …

can connected device devices employees employers etc everything history information it admins messages microsoft microsoft teams personal personal devices phone privacy search teams wifi work