When is Velocity calculated?

I have a code that instantly adds force to counter velocity during FixedUpdate.

However, I want this to be flawless, such that it never even allow the Velocity to take place.

When is Velocity calculated; before or after FixedUpdate? During FixedUpdate, would I have the required information from Velocity?

I believe that it is calculated after all FixedUpdate()'s are called. If you want to set velocity to a certain value, you can always set Rigidbody.velocity.