Can't add apps on my son's Apple Watch SE 2

He has watch, no iPhone. I went to my iPhone Watch App. Selected my son's watch ( which was paired already ). When I scroll down to try to add app, it gives me a message "Install new apps from the App Store on Noah's Apple Watch". That seems like an insane way to isntall apps as it requires a password every time.

I've seen people do it from their phones. Ideas?