Combat phase: attackers, blockers, instants and the Stack.

Hi friends. I have some questions about the combat phase on MTG and how exactly is the stack resolved around it.

I've been doing some reading and realized declaring attackers and blockers don't go to the stack (it's a turn based thing or whatever that means LOL) and I have some doubt about it. To be more specific if I can use instants to counter.

Case 1 - Player 2 uses instants

If in the combat phase of player 1, he declares a 3/3 creature as attacker, can player 2 use an instant [[Essence Extraction]] to destroy the creature?

What if instead of destroying the creature Player 2 uses [[Cruel Finality]] to lower the power and toughness of the creature and then declares a 2/2 as blocker, is that posible?

Case 2 - Player 1 uses instants

So this time Player 1 declares an attack with a 2/2 creature. Player 2 declares a blocker with 3/3, so Player 1 uses [[Essence extraction]] on the blocker. How does the combat resolves?

What if player 1 cast [[Cruel Finality]] on the blocker instead? Or what about casting [[Highspire Infusion]] on his 2/2 attacker after the blocker was declared?

Thank you for your answers.