Update() runs every frame, as fast as your computer can process. It does not run at a consistent rate.
FixedUpdate() runs once per physics loop, which you can set in the project settings. It can be called more of less than Update(), and not necessarily in sync with it. It is used for physics when calculations need to be simulated at a certain rate to ensure consistency. When using physics based movement, it is best practice to use forces inside a FixedUpdate() functions.
Time.delta time returns the time it took to complete the last frame. Use this to avoid tying your calculations to the computers frame rate.
void FixedUpdate () {
rb.AddForce(0,0,forwardForce * Time.deltaTime);
}FindObjectOfType<GameManager>().EndGame();