Photos keeps all your pics and vids in a "library" that the app accesses when you launch it. By default, Photos Library is stored in the Pictures folder of your home folder. You can open your home folder by choosing Go > Home from the Finder menu bar.
This library houses your albums, including the "hidden" albums. It is this Photos Library file that you want to copy to an external drive. Once you've copied the library to your external drive, you can simply double-click it to open it. By doing so, this tells the Photos app where your library is now located and sets that library as the default to open when you launch the app. (You can have several different libraries on your computer if you wish.)
When you're satisfied that your library is safely moved (and you have a backup of your library), then you can delete the original Photos Library from your internal drive and eventually macOS will make available the space that the library was occupying.
This is Apple's guidance for moving the Photos Library to an external drive:
Move your Photos library to save space on your Mac - Apple Support