Well it started a few years ago (maybe 5 or so) just as the simple, sole belief my mother's boyfriend would cough on purpose around me. Then that slowly turned into thinking everybody did, then over time (more relatively recent) that developed into the belief that pretty much my life/reality and every little thing was like that.

Might be worth mentioning I had just dropped out of high school, started losing touch with all my friends, and started smoking weed playing video games 24/7 too all at that time period.

Not sure if that's what you meant by where the beliefs came from but... The main reason my initial simple delusion about the coughing developed into such a widespread more complex belief I'm sure was due to things like getting my own apartment and living/being alone in my thoughts with a negativity bias for the last year and a half. Not to mention things like eating junk fast food 75% of the time and rarely ever going outside of my 10x10 studio apartment.