Finding original file on Mac for video shot on iPhone

Can you find it in the Library under Media Types ➙ Videos in the sidebar?

If you can just export the unmodified original to the Desktop if you want a copy of it.

One should not be mucking about in the library's package. It's not designed for that and can damage the library.

mstoreysmith wrote: Not planning any mucking, just copying.

Often people don't appreciate how sensitive the inner parts of the Library package can be. "Mucking" can occur accidentally, even without planning. This is, for instance, why we warn people against using untested (by us) 3rd party apps that claim to "clean" the Mac, or that "extend" the power of Photos. The few good apps, like  PowerPhotos ($30) , use Photos' own tools to avoid dangerous "mucking."

As Old Toad shows, Originals are always available through the Photos app. And you can always export the Original file exactly as it came to Photos with the menu File>Export>Export Original.

Open the library by double clicking on it. That will open the library with the Photos app. Then look in the Media ➙ Videos album.

You wouldn't be running a "referenced" library intentionally or by accident?

A referenced library is strongly recommended against.