Here's how a front end collision works. You crash and the car instantly sees it via crash sensors and the air bag module goes to work determining how to handle it, if it's an air bag worthy impact by the time you realize you crashed the car has done a bunch of stuff. As soon it gets a crash signal it unlocks all the door, turns the interior lights on, turns the hazard lights on, kills the engine, and disables the fuel pump. The air bag module deciedes if and how hard to set the airbags off, they are dual stage air bags meaning they can apply hard and fast or harder and faster. The drivers side it knows an adult is driving, and if the seat belt is buckled and how hard of a crash it was and deciedes how to set off the bag based on those things. The front seats have a pyrotechnic device in the reel, if it's buckled when you crash the seat belt sets off, it will retract about 8" and lock into place, the purpose of this is to pin your ass to that seat so A) the airbags can deploy as designed and not kill you and B) you stay firmly in your seat. As soon as the belt retracts. The pattern of the threads in the seat belt are designed to slowly stretch to help absorb the impact as you get pushed forward. The passenger side works the same except there are 2 more things it looks at, a belt force sensor and a PODs unit. The belt force sensor knows if there is any force pulling on the seat belt buckle, if there is the car knows the passenger is not sitting properly in there seat and may adjust airbag deployment strategy as it sees fit. The pods unit monitors the weight of the passenger, if it's a heavy adult you will get a harder airbag deployment than if it sees a 75 lb kid.
The seat belt deploys a pyrotechnic device and stretches out, they are a one time use item.
Most people don't pay attention but these cars are very intelligent when it comes to safety, it has earned it's extremely high crash test ratings.
