the top answer on https://www.zhihu.com/question/640078375/answer/3367442919 suggests it was based on the Minix 3 μk, but the bottom answer claims it's greenfield development.
(someone who cares more than I do could buy one of the devices listed in the middle answer and enlighten us as to what's on it?)
EDIT: looks not-minixy to me, according to Huawei's OSDI slides: https://news.ycombinator.com/item?id=41934156
In HarmonyOS NEXT, kernel objects are used as carriers for data transmission during IPC communication. The CAPABILITY SYSTEM ensures that only those with the capability to read from or write to kernel objects can receive or send messages through these objects. As a result, the content of messages cannot be accessed by malicious processes. DarkNavy ethical cybersecurity group in China tested Huawei's HarmonyOS Next custom kernel earlier this year with a report released in June. https://www.darknavy.org/blog/avss_report_kernel/ and the toolchain is exposed: https://harmonyoshub.com/harmonyos-next-leak-exposes-the-in-...