Can somebody help me understand this door chime wiring?
Since the batteries were there it probably did not have mains power connections. I have installed a few battery chimes and they last amazingly long time. Yours looks like a 2 location version (front door back door). I have not seen a chime that uses mains power that also required batteries , they may be out there but if you have a front door button and back door button I would bet that’s all there is no mains connections.