Where does Photos.app store its library and files?

After looking for it, I found it. Quite obvious actually, it is next to the former iPhoto Library.

~/Pictures/Photos Library.photoslibrary

Researching a little more, because both libraries report the same size, the import from iPhoto to Photos changed all original files to hardlinks. So either library contains a pointer to one file on disk, thus saving disk space.

For a full backup, I think it is sufficient to backup ~/Pictures/Photos Library.photoslibrary.

If you go to the following path /Pictures/Photos Library.photoslibrary, then right click on Photos Library.photoslibrary and select ShowPackageContents.

Then you can select the masters folder and backup the original photos.
Hope this helps.