After some research, I found that it was better to put all physics related code into FixedUpdate by saving input in a variable of bigger scope.

Although, for a single force (like a jump when a key is pressed), would I need to store the state of the jump keybind in Update and use it in FixedUpdate? Here is an example, I know my explaining is bad:

private bool jumpKey;

private void Update()
{
    jumpKey = Input.GetKey(jumpKeyBind);
}

private void FixedUpdate()
{
    if (jumpKey && other requirements)
        apply the impulse to the rb
}

This code does work, but it still raises [to me] the following question : does every single bit of physics need to be in the FixedUpdate method? If I want to execute the AddForce method when the player reaches somewhere like the top of a stairway, does it need to be in FixedUpdate?