By default, Azure AD adds the user performing the Azure AD join to the administrator group on the device. If you want to prevent regular users from becoming local administrators, you have the following options:
Windows Autopilot - Windows Autopilot provides you with an option to prevent primary user performing the join from becoming a local administrator by creating an Autopilot profile.
Bulk enrollment - An Azure AD join that is performed in the context of a bulk enrollment happens in the context of an auto-created user. Users signing in after a device has been joined aren't added to the administrators group.
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