Anyone can install an app onto an Android phone if they have the apk file, the program in other words.
They cannot ban TikTok. They can only restrict it from being distributed from the Google Play Store, and they can restrict other American entities from any business dealings with TikTok, and they will think up some bs law to do so.
But the actual details of restriction of the app itself, that is impossible essentially. The only way that can be done, is to tell Google to make Android not install that app. Not sure if Google can do that. Probably not.
The other thing they can do, is to declare the app illegal and anyone using TikTok to bust a move, would be in violation and could be arrested and sentenced to jail time.
So if there is a ban, this ain't over . . .