You can't as Microsoft designed those devices to only accept apps directly from the app store. However, there was one Win10 build (10240) that allowed the installation of Android apps via the developer mode. But since this is a five year old build i really wouldn't recommend using it as on your normal phone.
Let's face it, Windows phones are bricked as the app store is no longer available and many apps simply refuse to run (Whatsapp is one of those).