Here is a basic physics explanation for many of the improvements:
When you are in a car that is moving you have momentum = mass x velocity and kinetic energy = 1/2 x mass x velocity squared. When you stop such as in a crash you have to get rid of momentum and kinetic energy. Momentum is gotten rid of by the force of the crash exerted over time and energy is gotten rid of by the force of the crash exerted over a distance.
In the old days (before 1970's) car bodies were made of rigid materials. When a crash occurred the time and distance were short so the forces were high injuring the passengers. Without seat belts bodies are free to move and don't slow down with the car does leading to more injuries.
Fewer injuries occur if you are secure in the car and it takes more time to stop. Think jumping off of a wall. (Don't try this at home without supervision). If you keep your legs rigid the time is short and the forces on your bones are high. It hurts a lot! Bending your legs when you land increases the time to slow down and decreases the force. Air bags basically add stopping time which decreases the force on the humans. Break away poles and guard rails basically slow you down so you don't have as rigid a stop.
The kinetic energy is dissipated by breaking things. If the car is rigid it will be dissipated by breaking you the passenger. Making the front end of the car less rigid allows the energy to crush it instead of you. Front car frames crush more easily in crashes but the passenger frames are more rigid to keep you secure.