There is little water vapor in cold air at high altitude, partly because it's cold, and partly because the air density is low. Most of the water vapor comes from the exhaust. Hot H2O from exhaust condenses upon getting chilled.

Kerosine + Oxygen -> CO2 + H2O

No such thing. How do you think they'd keep it secret when there would have to literally be thousands of ground crew, airline personnel etc etc involved. They can't even keep a SCOTUS decision under wraps.

not chem trails - what you are talking about are called Contrails - short for "condensation trails" - you can read all about it with a simple web search:

Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust.