Stefan Mileschin

Technically, Android already blocks sideloading by default at an operating system level. The permission that’s needed to silently install new apps without prompting the user, INSTALL_PACKAGES, can only be granted to preinstalled app stores like the Google Play Store, and it’s granted automatically to apps that request it. The permission that most third-party app stores end up using, REQUEST_INSTALL_PACKAGES, has to be granted explicitly by the user. Even then, Android will prompt the user every time an app with this permission tries to install a new app.

https://www.osnews.com/story/140312/...ng-by-default/