I deleted the “built-in” camera from my Apple Watch, series 4. I want it back but I’m not successful.

There is no built-in camera on the Apple Watch. All there is is a remote to activate the camera on your iPhone, and the Camera app cannot be deleted from the iPhone. Is that what you are referring to?

Go to the app store on your watch and search for "remote" then scroll down the list,it is quite a way down and tap on remote to install it.