Using the same Apple ID on private and business iPhone
There are two separate questions here but hopefully they won’t get mixed if I’m careful about wording things:


Your company can install MDM profiles and unless they do, you only expose what addresses and traffic you send when you connect to their network or VPN or install their apps. Apple protects you since there are no hidden backdoors and if you have a company MDM - you will see it in the profile portion of the settings app and it has to list what it does.
You can choose to use a private AppleID in the App Store and that won’t let the business see what you do in the store other than seeing the side effects.


With MDM - they can see all apps installed - so if you installed a sports betting or game - they would see that IF they have MDM or if they see that app talking to the servers that run that app.
They can send apps to your phone on iOS 11 without needing you to be signed in, so it’s up to them if you should use an AppleID or just use MDM to push the “work apps”
Work can erase the device but not get data from it with MDM in general. Unless they get your passcode or you back up to their servers, Apple ensures data is secure with your password so unless they have surveillance or a key logger on a computer and capture your password - your password and account is yours and it’s your choice to use it on a work device or not.



So - If you really want to keep work from personal - sign up for and maintain a work AppleID so you don’t mix the streams. For many people, using a personal AppleID for just the App Store is super low risk and you can manage to not auto install apps and control what app history you wish to share with work. Without MDM, they can’t modify settings, wipe the device, lock the device, unlock the device. With MDM they can do those things, but you will see they can do it and ask about their policies so you can make a good choice.

I wouldn’t mix normal AppleID like messages or Photos or iCloud or GameCenter with work and pleasure - this answer just is about apps and access to app data. That second item could leak more data like contacts or conversations you would rather be yours.