Love this game. It taught me to look for a move like g4.

Also it was probably not objectively the best move (and definitely not Spassky's best game) but Tim Krabbé made a list of 110 most fantastic moves ever played, and he put Spassky's 16...Nc6 against Averbakh as no. 1 (certainly an unorthodox choice):

https://timkr.home.xs4all.nl/chess/fant100.htm

I know this is a dumb question even before I ask but I cannot figure it out: why do only some of the games on chessgames.com end in a final move that is actually checkmate, while others don't? On this one for example the final move says "Black to move" and you can move them around to get to a checkmate yourself...did they just shake hands and not play the final couple moves out?

Very new to the game, so have mercy on me please, but can anyone explain what's going on with these 0-0-0 and 0-0 moves? I don't understand how or why two pieces can move simultaneously.

Ohh, that is lovely. I had not seen it. Black does seemingly nothing wrong and is just in a world of hurt by the 12th move.