a 600 elo player checkmated me with Knight and Bishop

I think that he isn't a cheater

I just went on youtube and found great instructions. Now I do it easily. There is more than one tecknique to do it. Some are better than others. I go to the checkmates section of the study section and practice against the computer.

It does not matter if you never use it. What matters is the fun and confidence of learning it. I don't think I'll ever forget it. Partly that is because I enjoy the drill.

Also, checkmating with a king and queen:

There is the beginner way and the computer way. The computer way is over twice as fast. Move your king more and your queen less. That's the way I do it now. Bring their king towards yours, not towards the opposite edge.

I just use my queen as a rook to mitigate stalemate especially when you have a bishop on the board lol.

Sometimes, I’ll promote a rook and practice.

It depends on the position.

Something like (below) is easy to do accidentally

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But if they checkmated you starting from the position below, then they were cheating.

Just a reminder that 600 players make errors like this

It's not a matter of knowing the checkmate. A 600 rated player that makes the mistake above knows how pawns move, but they make the mistake anyway. 

Even if they know the ideas involved in the 30-something moves necessary to mate, they will fail to do it.

Show me the initial position.

If your king is already at the right corner and within 5 moves checkmates, it can happen.

From scratch, it is less likely than winning a Powerball.