How do iOS apps that use Contacts work on iPhone and iPad? The IDs for each contact don't match between devices

Question

Since the IDs for Contacts don't match up between iOS and iPad (at least in my mom's case, who uses iCloud to sync contacts between devices), how could an app possibly function on both devices?

My app allows the user to create groups of contacts, so I need to store their IDs in my DB.

I'm thinking that other apps might have done it in the past by using Apple's old AddressBook framework. I'm using Flutter's library contacts_service.