My Samsung phone came preinstalled with an unremovable third party app store. No root necessary. Maybe device manufacturers aren't considered third party?
Silent installation of applications without your confirmation or knowledge is a privileged permission reserved for preinstalled apps. So Samsung can provide a 3rd party store on their phones that can silently install applications on your phone while others can't.
This was changed in Android 12 (I think) where silent app installation is a grantable permission for other stores like F-Droid.
