If you mean modified as in installed another OS, barely any in this case. They're all non-Google certified operating systems so Play Integrity hardware attestation can force apps not to work on LineageOS, CalyxOS etc.

I mean Samsung and other brands are modifying the base Android a lot to add their bullshit UI and mandatory app store and apps. And it's still certified by Play Integrity otherwise all customer of phones other that Pixels would complain of apps not working. How is it different if Motorola officially sell a device with GrapheneOS installed stock as their version of Android ? It should be approved by Google and pass all Play Integrity check, right ?

The device / OS pair is certified. It has some requirements that if the OEM follows them they are able to be certified. For example, Google services available, many as privileged components. While we have sandboxed Google Play that is still not sufficient. Some users don't have a play services implementation at all. So while some OEMs make awfully changed Android releases, ours is left in the dirt there. We don't try for Google certifications.

Bitcoin & Android serve different purposes. Bitcoin is a store of value, Android for user experience. Both can coexist, each excelling in their domain.