Where does Photos.app store its library and files? 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.
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.