The very first Apple Watch firmware has been released as an OTA update. The firmware, which Apple is branding Watch OS, is actually a modified version of iOS 8.2, similar to how the Apple TV uses iOS firmware as a base.
The 38mm Apple Watch is ID’d as Watch1,1, and the 42mm version is ID’d as Watch1,2. As noted by developer Steve Troughton-Smith, the firmware is heavily based on iOS 8.2 and features a PowerVR SGX543 driver. As yo may recall the SGX543 was integrated on the A5 SoC’s that powered hardware like the iPad 2, and iPhone 4s.
Instead of running the SpringBoard, which is the launcher you see on iOS devices, the Apple Watch firmware is running Carousel. Interestingly, the Apple Watch firmware is broken up into two different builds—one for the 38mm watch, and one for the 42mm watch.
Nothing suprising – it’s running most of iOS 8.2 with Carousel instead of Springboard. Has a PowerVR SGX543 driver, so it’s A5-equivalent?
— Steve T-S (@stroughtonsmith) April 23, 2015
As of now, the OTA files are pretty much useless for most people, but if you want to peer into the various frameworks that Apple has included in the firmware, then it may be of some value to you.
If you want more information on the specifics of the firmware, then I suggest checking out IPSW.me. It’s a great site with lots of resources and download links.