Where did demons come from?
Thankfully, the Bible does not devote a whole lot of time to discussing demons and their activity, but we can trace their origin. Demons were once angels who rebelled and were cast out of Heaven. “For if God did not spare angels when they sinned, but cast them into hell and committed them to pits of darkness, reserved for judgment...” (2 Pet. 2:4; NASB). While many controversies surround the details of this passage, it is clear that demons were angels who sinned.
Regarding some of the minor points, the Bible just doesn’t make it clear. Personally, that is fine with me! The majority of the Bible focuses on God and His relationship with man.