When one downloads and then installs an application on Android from official sites or other unofficial sites, they are actually installing APK ion the device. It is possible for a person to install APK files directly in devices. This can be done without necessarily downloading them from any network. You can use a desktop computer and use different communication programs.
You can also use a process that is referred to as side loading. Usually, this ability to install from sites that aren’t official or from the file manager or direct from a desktop comes disabled because of different security reasons in many of the Android devices that we have. You can enable this by simply changing the unknown sources settings within the menu.