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The device would have to be jailbroken right? These apps are (obviously) not in the App Store, I mean one of them is a cracked WhatsApp ...

No, you can distribute custom managed apps through Apple's MDM programme. https://support.apple.com/en-gb/guide/deployment/dep575bfed8...

Sorry yes I meant for personal devices. These are designed to be deployed under MDM on corporate devices

You can put personal devices on an MDM, many have special modes for this too.

These are all high profile, high net worth individuals. Their 'personal' devices will all be managed by MDM as well.