It's a fork of the Android Open Source Project (AOSP) with major privacy/security improvements and alternatives to Google apps/services. The massive set of changes needs to be ported to new major versions of AOSP.

The apps also need to be updated to the Android 17 target API level but that can happen over several months following the OS itself being ported to it. The app aspect is something all Android developers need to deal with due to new target API levels bringing backwards incompatible improvements.