Huawei has reportedly stopped its smartphone production after US blacklisting

I stopped reading when I ran into this:

Google’s Android smartphone operating system is the backbone of Huawei’s massive smartphone business — but in the next three months, it will stop getting updates on Huawei’s devices. This will give time for Huawei to prepare its own OS.

I can't imagine how it would be even remotely possible to write an operating system in three months!

If all they mean by "OS" is to replace the closed parts of Android that they lost access to, three months still probably isn't anywhere near enough (unless they started working on that a long time ago and happen to be nearly finished with it).

Since Google is/was already banned in China, how did phone manufacturers circumvent that in order to use Android in the past?

Did they just sort of turn a blind eye towards it or something?