If you only install the GooglePhotos app, you can manage them in that app, and also load them to Google Drive. The images in GooglePhotos are not automatically in Drive. You can export them to the Drive app, though, which isn't necessary, imo.
This might sound confusing, that's what it looks like in my apps. The images from Google Drive can also be displayed in GooglePhotos (a setting in the app).
Google Drive, on the other hand, doesn't show the images from GooglePhotos by default. You'll have to search for them (literally).
Both apps are connected to each other, somehow, but each has it's own purpose.