The Bible teaches that physical death is the separation of the spirit from the body:
James 2:26 For as the body without the spirit is dead, so faith without works is dead also.
That separation of the spirit from the body is not some mechanistic, automated, natural process that impersonally happens on its own. God takes away the spirits of living beings so that they die and return to dust:
Psalm 104:29 Thou hidest thy face, they are troubled: thou takest away their breath, they die, and return to their dust.
Ecclesiastes 12:6 Or ever the silver cord be loosed, or the golden bowl be broken, or the pitcher be broken at the fountain, or the wheel broken at the cistern. 7 Then shall the dust return to the earth as it was: and the spirit shall return unto God who gave it.
Daniel 5:23 But hast lifted up thyself against the Lord of heaven; and they have brought the vessels of his house before thee, and thou, and thy lords, thy wives, and thy concubines, have drunk wine in them; and thou hast praised the gods of silver, and gold, of brass, iron, wood, and stone, which see not, nor hear, nor know: and the God in whose hand thy breath is, and whose are all thy ways, hast thou not glorified:
Luke 12:20 But God said unto him, Thou fool, this night thy soul shall be required of thee: then whose shall those things be, which thou hast provided?
Physical death is God's punishment for sin.