How do you import old photos from other non-apple devices into Apple Photos?

Are you asking about Photos on a Mac or Photos on an iPhone? You are asking in the Photos for Mac forum, but your device is listed as iPhone 12 Pro?

When you want to import from older other devices, you may need to convert the photos to a compatible format. Not all older formats are supported on macOS 11 or iOS 14: About legacy media in iMovie for macOS - Apple Support

See: Import photos from a camera or phone using Photos on Mac - Apple Support

Depends on the device.

Normally you can copy the photos to your hard drive from a memory stick/card, or direct from the camera or whatever method is applicable to your device.

Then you go to the file menu in photos and select import, navigate to the photos and bobs your uncle. :-)

(In many cases you could import directly from the memory card or camera if it appears as a drive in finder)