It doesn't detect the act of recording live, it detects that a piece of media was obtained via recording. So, you can still point a camera at the screen and obtain a video file without any disruption to the original signal. However, that file won't play properly on Cinavia-enabled devices.
It's not clear to me how widely Cinavia is actually deployed. The Wikipedia article hasn't really been updated in over a decade, and that's where I'm getting my info from.
However, the detection and enforcement can theoretically be done by any device or software that has access to the audio signal. The monitor, the GPU, the playback software, the operating system, etc. could each individually decide not to play the file, making it not work. Some of those can be bypassed in various ways, some can't. But instead of computers, there are smartphones, commercial media players/receivers, and televisions/projectors, which seem the most likely places to target for enforcement, and those would affect most people.
Nevertheless, I do wonder how real this actually is. Again from the decade-old Wikipedia article, it seems like Cinavia was meant to target both recording devices and playback devices. However, the Aurora theater shooting happened not long before the article stopped getting meaningful updates, and I wonder if public safety concerns stalled its deployment. Also, the article mentions that people were finding ways to remove or neuter the signal. I also didn't encounter any problems with what I assume to be protected media (a 4K movie and a 1080p TV show), either recording my screen with my Android phone, nor with playing it back on that phone and with VLC on my Windows computer with an nVidia graphics card.
In practice, only licensed Blu-ray players (hardware and software) implement Cinavia. While there was an idea/fear that an OS can implement Cinavia system-wide, that did not happen (yet).
Depends who makes your computer, phone or TV and the licensing etc. The tech is perfectly capable of stopping that. The device detects the Cinavia watermark and simply silences the audio after a few minutes.
I don't know whether streamers use it but it was widely deployed in the era when movie piracy revolved around making pirated Blurays. For instance the PS3 would silence the audio on a burned Bluray that had a theatre or TV cammed title protected by Cinavia on it.
A lot of this is about catching the fat head though. People who play videos using some hacked up VLC on Linux don't bother the studios, they're long tail and don't make a revenue impact. They're after the ordinary people who want to watch pirated stuff on a regular home cinema system.
