Hi There. So, firstly, you can browse through here: https://developer.apple.com/documentation/devicemanagement/commands_and_queries to determine what your employer can and cannot see
However, keeping this much more simpler.... photos, whatsapp chats, emails, messages are NOT available to any MDM. This is the user data most likely to be available
Persistent device IDs (serial number, MAC address, IMEI)
Phone data (amount of data used, phone number, IMSI / ICCID, etc)
apps installed. They cannot see if these are in use, however
If you are worried about any of the above, I suggest you have a conversation with your employer and discuss your concerns. MDM has been around for nearly 20 years and is used by millions of people around the word. It cannot be used for getting access to your personal data. And it was never intended to do so.