The US tries to keep its surveillance secret. Seems to me that this makes it unlikely that they'll backdoor apps, because random hackers might reverse engineer the apps and discover and publicize the backdoors. Instead, the US backdoors communications lines and data centers, or uses existing security holes to break into targets.
China doesn't keep its surveillance a secret. They don't go out of their way to publicize it, but they don't seem to care if people know it's there. A backdoor in an app would fit their style better.
China doesn't keep its surveillance a secret. They don't go out of their way to publicize it, but they don't seem to care if people know it's there. A backdoor in an app would fit their style better.