"HarmonyOS NEXT (Chinese: 鸿蒙星河版; pinyin: Hóngméng Xīnghébǎn) is a proprietary distributed operating system and a major iteration of HarmonyOS, developed by Huawei to support only HarmonyOS native apps."
HarmonyOS NEXT is based on an open source core, OpenHarmony [0], with proprietary additions.
So, not hugely dissimilar from iOS (lots of bits of which are open source, most significantly the core of its XNU kernel) and Android (considering that the proprietary Google Mobile Services is de facto a mandatory component)