How does photo/video recording work until the button is pressed (Live Photo, etc.)?

Question

Algorithms

Interested in the process of recording video / photo before pressing the record button.
For example, Apple's Live Photo (1.5 sec "video" before pressing the button and 1.5 sec after) and the function in the DVR (30 sec before pressing the button / collision).
Google didn't help.

Answer 1

The video is written to the circular buffer, new frames are overwritten on top of the old ones. At the moment of pressing the button, what was in the buffer is saved to a file. This will be those 1.5-30 seconds of video before clicking.