> How does this work when apps use GPLv3?

Android Open Source Project is mostly Apache licensed, it runs on the Linux kernel which is GPLv2.

This situation with the firesticks is essentially the same play that TiVo pulled way back when, and the GPLv3 is supposed to counter.

Amazon has moved away from Android for the new Fire devices. It is a Linux custom distro with React Native and Web for apps.

As for Android, I would assert that Google has successfully removed everything GPL related from early Android days, the only thing left is the Linux kernel.