While black is somewhat winning with other moves, can you find the only move that truly solidifies a completely winning endgame for black?

Trouble understanding chess books

I understand chess notations but what is this? Why are the moves separated in 4 columns and why does it only have white moves? (Book : Pocket encyclopaedia of chess openings - Jon Speelman)

My first brilliant and it was intentional!

First intentional brilliant, white took the bait

I just got an estimated 1200 rating in a game

I’m 600 How accurate are these? Of course it’s just one game, but I was very surprised to see this.

Made my first brilliant move!

What's wrong with Qxh1 here?

I played Nc3 before seeing Qxh1 then went into panick mode, but my opponent played Qd8, and apparently Qxh1 isn't a thing. But I can't see how to punish it.

Do you see why this move is so destructive for black?

Why is this a bad move?

It says i missed a fork but i created one

I believe this is what you call, a Smothered Mate

What tactics should I use here?

This is a practice game against a bot so I hope I'm not cheating! I struggle a lot with middlegames and often find that I'm just not sure what to do in standard, common positions like this one. If there are any experienced chess players here, could you help me know what to look for to make progress? I'd like to use this game as an example since I'm stuck. Thanks all!