You use stock Android Open Source keyboard, not closed-sourced Gboard? Can you type in Chinese?
China doesn't uses Google services. Am sure they can type in Chinese.
Those Android phones are loaded with their manufacturer's Chinese IME, not a completely open-source one.
CJK languages need language-specific AI conversion engines for any kind of typing. Which OSS versions exist for Japanese, I don't know for Chineses(CN/HK/TW - IIUC they're slightly divergent beyond fonts).