Can someone explain this old switch wiring?
The looped through wire will be the live feed, and it continues on to power another circuit, probably the other switch.

Motion sensors usually require power of their own to function, and in newer installations there'd be a neutral available to provide that circuit - as there appears to be in your box. Check the instructions that came with the sensor as to the wiring but this looks like most of the dimmers and sensors, the green wire provides the circuit by connecting to the ground along with the bare wire. Not exactly per code, but that's how they've been sold for years.