Search by metadata in Photos app?

Same as you do all metadata searches in iPhoto and Photos - Use a smart album - file menu ==> new smart album

for cameras use the Camera is [check the one you want from the drop down list]

I'm OK with using "Show package contents" on the Photo Library in the Finder and searching from there

Don't - this is not the supported way to do anything and is extremely dangerous - the smaller mistake can damage or destroy your library and cause data and or photo loss - there are no user serviceable parts within the Photos library package

LN

Create a smart album like this:

And as Larry pointed out, never go into the package via the Finder. That's a receipt for disaster.

Both answers were very helpful to me, but when I set up the smart folder, I did not get a pick list in the field for Camera Model and, if I copied the name of the camera from Get Info, it did not show any photos. Am I being impatient? Should I just wait until it populates? (I have >80,000 photos.)

Belay that: I got the answer -- for some reason, the Camera Model I have should not include the name of the maker, Sony!

The field "camera model" may initially come up as blank. Then just click it and hold it and wait. it should populate slowly, while Photos is searching your library for all available camera models used for your photos.