What exactly is recorded and what is not?

This again depends on the camera and software system. A fundamental distinction is made between permanent recordings and live images, which are stored entirely on hard drives, if at all.
This is often the case with companies and companies with permanent video surveillance so that in an emergency complete recordings are available and live cameras that record individual videos through a specific trigger.
This technique is more commonly used in private property surveillance, which often records when motion is detected once the system is armed. In this way, only individual potentially suspicious clips are saved.