One byproduct of combustion is water vapors. As molecules get rearranged during the combustion process you are using heat to make the gasoline (a HYDROcarbon, i.e. hydrogen and carbon) react with OXYGEN. So some of it comes out with the hydrogen binding with oxygen creating H2O (water, hydrogen and oxygen) , and others are CO or CO2 (Carbon and oxygen) the leaner the mixture the more likely you get the relatively harmless co2 instead of the deadly CO because the more oxygen rich mixture allows for mpre complete combustion with two oxygen molecules to bind to it making co2 instead of just one binding to it making CO.
Anywho, those water vapors condense when the exhaust and engine cool into actually liquid since colder air can’t hold as much moisture as the normally hot exhaust. When you start the car that condensed water gets blown to then end of the exhaust and drips out.
I am probably not 100% accurate on the chemistry, I am just a firefighter that likes to also work on cars. I understand how fire works.