I searched whole internet and no answer. Self infection apparently is out of question, so WTF it comes from?
Common cold virus is thought to have originated from Birds 200 years ago? That's the best I can come up with. https://www.sciencedaily.com/releases/2008/11/081120073115.htm Likely animals, plants, insects. A popular misconception is that you will catch a "cold" from cold temperature. But the reality is that virus's just survive longer in the cold allowing you to acquire it. The temperature being cold itself will not get you sick. That's all I know.
Usually people are either infected through direct contact, breathing in aerosol droplets in the air, or through contaminated fomites.