Your experience with AliExpress is different than mine. However, I bought a couple of Mavic 3 batteries that no longer work after a DJI firmware update. But they worked perfectly up until then. Certainly not Aliexpress' fault. My other example...
Depending on what you want to install. You can download just about apk from some websites. I use apkpure for most of them. When I was using Litchi, I used the Amazon Appstore, but Amazon is killing that off soon. You'll have to enable Developer...
Yes, each drone has a different operating firmware. If you had a Mavic 3 Classic like me, you would choose that drone and you'd get a popup saying to switch firmware.