For those interested in integrating your watch via Linux with your phone, here are some resources for Linux development
First learn how Linux file system works
Here is good article to read more on Linux filesystem.
Second learn how Linux Kernel works
All Linux OS have one thing in common, that is Kernel. In other words, if you want to customize a Linux OS to suit your requirements, you need to know and make changes to Kernel. Below are good articles to learn more. Make sure to backup from your system data, so if you mess up with your OS Kernel, they might be no way to recover them.
- How Linux Kernel manages hardware memory
- How Linux Kernel creates and manages OS processes
- How Linux kernel performs task management
Third, pick a Linux OS
There are two types of Linux OS: common and specialty. The common ones are like Ubuntu, Debian, etc, whereas the specialty ones are designed to solve a specific problem. See below links for details