How much of Huawei’s OS is reverse-engineered Android code?
Why would they need to? Android ASOP (the public domain version) is freely available even for use in commercial devices. The question is would it fail without access to the app store...
Considering how many use downloaded APKs in Asia, I suspect that it's not as big an issue - a far bigger issue for app developers as they won't have the opportunity to draw revenue from these phones. Without the Play store, that avenue is almost entirely cut off. But as Amazon has demonstrated, an alternative app store can succeed, and they certainly have a large enough audience to field their own app store.
Android is open source, so it’s not reverse engineered anything it’s just engineered. All the source code is available online.
I’m sure they’re just building their own flavor of Android without any Google components. There are million of these flavors already available that you can install on existing Android phones with unlocked bootloaders. I have an old Google Nexus 9 tablet with a custom ROM on it that has absolutely zero trace of Google or any of their services installed. It’s basically just a generic Linux tablet.