I couldn't find a link to the source code. Then I found this: >KaiOS is based on the Firefox OS open-source project and we are committed to abide by the rules

I couldn't find a link to the source code. Then I found this: >KaiOS is based on the Firefox OS open-source project and we are committed to abide by the rules of the applicable open source licenses. Therefore, we’ll make the source code available to the extent required by the applicable open source licenses. Haha. Good luck with your OS. Next.