Matthew Panzarino, writing for TechCrunch:
The Touch Bar itself runs a lean, modified version of watchOS, likely because that’s what the T1 needs to run it, send data and render images.
What Apple is doing with this hybrid configuration is very interesting. Instead of ‘switching’ to ARM chips, it chose to build custom chips for a precise task and use them specifically for it. Just as the Intel chip is good at the heavy, multi-threaded lifting of OS X, the T1 was made for the security of Touch ID and the lightweight lifting of rendering Apple Pay dialogs securely.
This is clever implementation by Apple—instead of making a touchscreen Mac, Apple added a touch-based component that works with macOS and its underlying architecture.
This last part is interesting indeed:
What other things are enabled by this onboard T1 in the future will be interesting to see. I’m also more sure than ever that an ARM-powered keyboard for iMacs and Mac Pros with Touch ID built in is on the way, soon.