How does a modulating gas burner adjust the fuel mixture?

My new Burnham Alpine gas boiler uses a variable speed blower to modulate the flame output down to 1% increments. How does it adjust the gas flow to keep the proper fuel/air mixture at all flow rates?

An old car does this with a carburetor. A new car has a mass air flow detector feeding a computer which meters the fuel injectors.

 

A negative pressure gas valve with a computer controlled combustion blower, just like your car with a less dense fluid.