

As someone who uses a Xiaomi phone, that’s not true.
It was a faff getting it to install any random apk I found of off the Internet, but it was possible. Did involve holding down a graphic for 10 seconds as part of the unlock procedure.
But for now at least, you can still install any unsigned apk without going through a store.
USB adaptor
If context is unclear might add a “mains” or even “C” in the middle as needed.