All my searches on this topic just return results about how to disable (built-in) apps on Android, which is the opposite of what I want. I want to know how it is that some apps do not allow disabling or force stopping, and I want to know how to make that the case for certain downloaded apps. For example, Norton App Lock is supposed to prevent certain apps from being run on the device, but what good is it if you can simply disable Norton App Lock and then open any apps you want? I notice that some apps prevent force-stopping, and some prevent disabling. I want both to be prevented. This is for Android 6 Marshmallow.
If you have root, you move the app to
/system/priv-app; privileged system apps can't be disabled through normal user methods.
Otherwise, it's up to the application to support Device Administration. Apps like Norton App Lock should be implementing this if they don't already, since this kind of thing is exactly what it's designed for. Registered and activated device administrators can't be disabled or uninstalled until they are deactivated.