in Photos, can I find the serial number of an iPhone or camera which took a photo

Photos retains all the information in the metadata that is recorded by the camera, and it shows you a portion of that information in the Info Window. Even if it isn't shown there, when you export the original picture it will still have all the original information in the metadata. This includes stuff like this:

You can see the make of the camera, the model, the version of the software, and all sorts of stuff. But not the serial number.

If the manufacturer of the camera wished to include the serial number, I'm sure that they could. But I've never seen that. You can imagine that such information included in pictures that might be shared could be a bridge too far.

I used the program GraphicConverter to bring up the metadata above, but there are lots of apps available that let you see all that stuff. Again, Photos retains all that, but it pulls out and shows only the the stuff in the Info Window that they think will be useful to most users.

If you need to be able to distinguish two cameras that you have, many cameras let you include copyright information in the metadata. You could put a name or a serial number in that field. Check the camera's menus.

.