AFAIK it's mostly not open sourced, but from what I recall the H1/H2 chips used in the devices improves and augments a lot of functionality. It includes a bluetooth modem among other things, and the result is much improved performance and pairing. On the software side I think (possibly also facilitated by the chip) Airpods also utilize a feature for switching between devices using your Apple ID. I think Apple automatically syncs bluetooth settings between your devices.