I'm unaware of any actual hardware backdoors introduced by primary manufacturers in practice, especially those installed in Xiaomi hardware.

There are accounts of interdiction and post-manufacturing compromises, such as those revealed in the Snowden leaks (2013) and most recently by reporting about the Israeli security services' sabotage of Hezbollah pagers and 2-way radios. These projects didn't expose primary manufacturers to the reputational and legal risk of association.

Until there's evidence of backdoors implanted by these companies, it's better to adopt a "trust but verify" approach and use standard layered security practices to detect intrusion. Conventional network intrusion and insider threat campaigns carry less risk of failure and are simpler and cheaper to execute.