I switched from iPhoto to Photos and now one feature that was important to me is missing: "Show referenced files in Finder".
For example, that feature was useful to me for selective sharing. Also for other things, like complete Library backups, I think it is very important to know how Photos.app works and where it stores the files.
Best Answer
After looking for it, I found it. Quite obvious actually, it is next to the former iPhoto Library.
~/Pictures/Photos Library.photoslibraryResearching 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.