Compilers and virtual machines are covered in junior year CSE courses. It's not rocket science. The main difficulties in such systems are designing the functional requirements, but in case of making something to just run Android applications, the goalposts are set.
To give you a simple example, imagine Huawei writes a function that makes use of another function in the AOSP. Now Google comes and changes the behavior of the AOSP function, thereby breaking Huawei's function. What can Huawei do?
1. Rewrite the Huawei function: can be very costly if there are lots of such dependencies
2. Do not take the Google change: now the Huawei OS behaves differently from the Google OS and third-party apps will run into problems
