Does Photos make its own copy of the photos that are imported to it?
There is a way to tell Photos to make a copy when importing or not.  In Photos --&gt; Preferences check Copy items to the photo library. 

Photos' default behavior is not to include original images in imports. It keeps links to them, so you can use Photos to edit or catalog them but still keep them where they are.

It reduce the size of Photos' library but if your goal is to clean up folders and disks with lots of photos on them, it can get confusing. To put everything actually in your Photos library, make sure to modify Photos' preferences to copy imported files to your Photos library. There's another important reason to do this if you plan to share those imported images on other devices: Only items copied to Photos' library will be uploaded to iCloud Photo Library. link