Where does Photos.app store the iPhone videos I've imported?
I figured out the answer by going to Terminal and doing find ./ -name *.MOV -print. 

The Photos Library is different from the iPhoto Library. If you've upgraded from iPhoto to the new Photos app, then for any video files you've imported, you can find the originals at:

~/Pictures/Photos Library.photoslibrary/Masters  


This way, you don't need to export your video files; you can just copy them.


In Finder, go to Go > Go to Folder 
Enter ~/Pictures/Photos Library.photoslibrary/Masters
Navigate to whatever video files you want and copy them to your destination.