Does the Angel of Death act alone, or does he have assistants, and is the soul the sole cause of life, or are there other factors that affect its continuity?

The Angel of Death is one, and he has assistants. Allah says: "Say: The Angel of Death, who is entrusted with you, will take your souls; then to your Lord you will be returned." (Surat As-Sajdah, 32:11). As for his assistants, it is as Allah says: "until, when death comes to one of you, Our messengers take him, and they do not fail." (Surat Al-An'am, 6:61).

A living being dies when its blood runs out, because blood is the essence of its life, or if it sustains a fatal injury. As for the soul (ruh), it is a divine secret that Allah has placed within the living body, and with it, He has entrusted life. Without it, the body becomes an inanimate corpse. It is among the matters whose knowledge Allah has exclusively kept for Himself, as Allah says: "And they ask you concerning the soul. Say: The soul is by the command of my Lord, and you have not been given knowledge except a little." (Surat Al-Isra, 17:85).