I wonder how iMessage goes in China? That’s end-to-end encrypted. Of course you have the loophole with iCloud, but if that is disabled or Advanced Data Protection is enabled (provided you can do either in China), the only way for them to see messages would be to compromise one of the devices. Or maybe it just isn’t allowed in China.
And this also highlights the downside of only being able to install apps from the App Store. Though I guess there’s nothing stopping China from blocking access to Signal’s website directly either.