Where collision impulse happen?

Hello everybody.

Im trying to understand how physics work at low level to see if my gameplay idea is viable. But, im not having sucess.

Someone can help me pointing a direction where i can find the exactly moment in code that a pair of objects collide and then, the physics simulation add impulse to both ? (both are movable)

I ill try customize the impulse of one object in this exactly time (before impulse is applied).

Pointing me a way to find it in code ill help my learning curve. Thx.