I mean a distaste for capitalism puts you in good company here but Mao’s China is one of the shittiest examples you could pick as an alternative, that’s one of the governments the right throws around as a stereotype for why they say communism is horrible and they do that with good reason.
They’d be doing this change on their TVs, in principle this would be no different from displaying the settings menu that comes up when you press the * button, no tampering of the HDMI signal required. HDMI inputs are their own “channels” in the UI so it’s pretty trivial to put ads on the screen periodically when one such channel is open.