Where can I find details on the physics engine?

I have just one question, everything else I write is just to satisfy your curiosity. My question is: How can I find out exactly how Unity evaluates forces, velocity, friction and drag to calculate the next velocity and object position?

I'm having trouble with object movement. The variables at play are drag, friction via PhysicMaterials and using a lot of rigidbody.AddForce(dir, ForceMode.VelocityChange). The biggest problem is that doing this work in the Update loop yields different results for different framerates, but in theory it should not, because logic like this is as simple as it gets:

 velocity += deltaTime * acceleration

The above is a great example because it's easy to debug. The problem is that the velocity getter is not updated until the FixedUpdate loop, so adding several different values to velocity will just continually use the old value.

 rigidbody.velocity += explosionA;
 rigidbody.velocity += explosionB;

The above does not yield the expected result, because velocity is precalculated. It comes out to be the same as the following:

 Vector3 cachedVelocity = rigidbody.velocity;
 rigidbody.velocity = cachedVelocity + explosionA;
 rigidbody.velocity = cachedVelocity + explosionB;

My bigger problem is that it's hard to get movement speed and acceleration quite right, since the game is using a mish-mash of built in physics and manual controls to get everything "just right".