Of course WeChat dominates china when they ban other apps from even operating. WhatsApp can’t operate there. Facebook is entirely banned in fact. Twitter is blocked as well.
Amazon failed to get a foothold due to complex regulations restricting them, which forces them to shut down their marketplace there.
So you can’t really compare that to a much freer market.
While not impossible, you have to try fairly hard to fuck up your phone like this. I’d be actively impressed if your story is true (particularly as you used the plural), and if so I’d like to know what they were specifically trying to install that fucked up their phones.
It’s just statistically more likely they downloaded a malicious app from the Play Store than had any chaos side loading.