I successfully created some demos using LAMW on Asus Zenfone 2 Intel Atom Android 5 and Redmi 4x ARM Qualcomm Snapdragon 435 Android 7 in the past. I now don't do Android programming.
how to know what phones are supported
I checked it by comparing the Android version installed by default on the phone and the API level l was going to use on LAMW. Here is the list of API levels and Android versions:
https://developer.android.com/studio/releases/platformsis it somewhere possible to see exactly what is the difference between API version and what exacltly is supported currently with LAMW?
For the difference between API levels, see the link I gave above.
For what APIs are currently supported, can be seen/chose on the LAMW API configuration.
As well what emulators are supported with?
Sorry, I cannot help. I tested the apps directly on my phone. Emulators were too slow. I heard Bluestack is better, but I haven't tried it.
Note:
LAMW is simply a tool to combine Lazarus binary with Android tools. You will get more useful information by reading or doing research on Android documentation. Google provides a well-documented massive information about Android programming.
https://developer.android.com/