It uses a package folder; if you do "show package contents" you can find the masters inside. Photos is non destructive, so those should be the same as when you copied them in there. Don't change them, but you can copy them out for that purpose. Use an alias or symlink if you want to make access easy.
For the future, though, perhaps you'd be happier referencing photos rather than storing them in Photos managed library. Your choice. Just uncheck the "copy" on import option in prefs. Then Photos will leave your images in whatever folders they were in when you imported.