how do i move apps back to my homescreen

I don't understand how to move it to the home screen. when I press on the app, it opens up a window that can't be moved to the home screen.

To move it back to the home screen, swipe all the way left until you get to the App Library. Find the app in question, press and hold and then move it to a home screen. If you wish to delete the app, press and hold and then tap on Delete App.

You do not press and release. You press and hold, and once they begin to shake, you can move it. You can read this, Organize the Home Screen and App Library on your iPhone - Apple Support or you can watch this Apple Support video.