What they actually did
- Three candles, and no other light on the first scene described.
- The performer was asked to lean over the table and write very close to the flames, so that her face caught enough light to expose.
- Shutter at 1/50, which is the ordinary setting rather than anything unusual.
- Aperture around f/2 to f/2.8, varying by shot.
- ISO close to the camera's native sensitivity, with one camera's second native ISO of 3200 doing the work on some shots.
The reply named the bodies used as a mix of an Ursa Mini Pro and a Pocket 4K, and said the settings were nothing extreme. That is the part worth carrying over to any camera: the exposure came from staging, not from pushing the sensor.
Why the background goes black on its own
This came out of the exchange rather than the first answer. The asker went away and read about the inverse square law, came back with the conclusion that a light close to the subject leaves the background darker while a more distant light lifts subject and background together, and the channel confirmed the direction by pointing to their own explainer on it.
That is the whole mechanism. A candle is a weak source, so it has to be close to expose a face at all, and being close is exactly what makes the light fall away sharply behind the subject. The black background is a consequence of the staging rather than a separate lighting decision.
What this does not tell you
- The exact settings were not recalled precisely, and the reply said so.
- The described setup is one scene from one production, not a tested method.
- Nothing in the thread covers how the shot was graded, if at all.