There are no details really as to how Xiaomi censors those terms. If one does not use the bundled-in browser / app-store, I doubt Xiaomi can censor anything at all in other browsers unless they MiTM with client-cert. OTOH, many popular non-browser apps (at least the ones that matter) pin certificates, so even Lenovo-esque shenanigans wouldn't work [0].
What can they possibly be doing in the firmware or the ROM to break TLS (and other such authenticated key-exchange protocols)? The only thing I think of: Injecting a compromised https stack in to an app's classpath / ld_library_path. This may sound ambitious, but the Android modding community already uses such runtime swappers to great affect [1][2].
[0] https://news.ycombinator.com/item?id=9072424