Access folder in which Photos stores its files
Launch photos, click on the Photos in the top navbar. On the dropdown, click Preferences > General and click Show in Finder. This will open the library. On top of the show in Finder button, there is the path to where the Library is located in.

Update: As Tetsujin said, the Library folder appears as a flat file rather than a directory. So access it by Right clicking > Show Package Contents