Decrement with --

Hello, So I'm new to unity and I was watching some tutorials. I used this code on double jumping by Blackthornprod in youtube.

if (Input.GetKeyDown("space") && extraJumps > 0)

     {
         extraJumps--;
         rigidbody.velocity = Vector2.up * jumpForce;
         PlayerAnimator.SetBool("isJumping", true);
         CreateDust();
         Debug.Log("Triggered");
     }

It's working on my part but I don't really understand it. My question is,

1. why does the variable extraJumps only decrement on the second time I jump? When I hit the space twice, the "Triggered" logs twice but the decrement only executes once.

2. As an additional problem, my isJumping bool only sets to true on the second jump as well.

3. When I add conditions in this particular if statements like && isDodging == false && isGrounded == true the second jump doesn't work anymore

I'm sorry if this is stupid but please help me. Thank you!!!