As you need to recompile the app instead of directly running it I guess they have made wrappers that implement the native Android and IOS APIs on top of the APIs for Universal Windows apps? That would be a massive effort.
The other question is whether they recompile Java/ObjectiveC to something else (C++ or .NET) or whether they are running these languages natively and providing interop to their native SDKs.
For Android they are just using LLVM to generate the binary and deploying it to their emulator (hyperv based I believe).
When delivering to Windows they use an android subsystem and a container to run it. It is a quick and dirty approach, you still would need to put some Windows specific code to access to Windows only SDK such as inking API or Cortana.
If you are creating an app from scratch it seems like an universal app with c++ shared code is the way to go
As you need to recompile the app instead of directly running it I guess they have made wrappers that implement the native Android and IOS APIs on top of the APIs for Universal Windows apps? That would be a massive effort.
The other question is whether they recompile Java/ObjectiveC to something else (C++ or .NET) or whether they are running these languages natively and providing interop to their native SDKs.