Platforma might be a bit more popular than watchOS. We know about 1 link to it since March 2021 and only 1 link to watchOS. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
You can find the list of compatible watches all the way at the bottom of this page: apple.com/watchos/watchos-7 (it's right above the SDK section) Hope that helps clear up your confusion! Source: about 3 years ago
There are some decent starters in the community (including some really great OS-native resources), but another option is purchase something like Platforma if you're looking to make interactive wireframes - https://platforma.ws/. Source: about 2 years ago
InfiniTime - InfiniTime is an open-source, C++-based firmware OS specifically made for the PineTime smartwatch.
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
wasp-os - Firmware for smart watches based on the nRF52 family of microcontrollers (including Pine64 PineTime)
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
InfiniLink - iOS Companion App for InfiniTime and the PineTime smartwatch
Method Wireframe Kit - Find your method of being incredibly fast in prototyping.