Whatsapp it's blocked countrywide so it's not going to be a problem (although I found out that some cell stations nearby tourist points aren't blocking that)
In China is easy. Once you give some millions to tencent/Alibaba/Baidu/bytedance (or having the government force them to do so) for porting their apps, it's all a Chinese citizen needs.
There are many apps that are actually "mini apps" (a WebView inside tencent WeChat)
When you enter China you need to register with the customs using said "mini app" in WeChat
90% of users exclusively use WeChat, alipay, Baidu, taobao, amaps, douyin (TikTok), xiaohongshu.
Maybe some could find useful to also have Weibo and pinduoduo (temu malware) but the "must have" apps are really a handful, it's not like in the west
Tizen was written from scratch. From what I understand HarmonyOS is Android... at least it was forked from Android. oh shit, I talked before reading.. sorry for that! :-D
Apps typically don't interact with the kernel. They interact with a C library like glibc, bionic or musl. So long as the kernel has a POSIX interface you should be able to compile the same app on different kernels. We do this with apps that run on Linux, BSD and QNX for example.
As for Android apps, you could extend that to running the Android runtime on top of different kernels. We did that with the Android runtime on QNX for BlackBerry 10 and we did it in a way that didn't even require recompilation. That said Wiki says they don't support Android apps. So if you meant Android apps they simply don't.