what Engine does chess.com use, and how do GM’s fair against?

If you mean the computers in live chess, see the recent article about Hikaru versus the many levels of Komodo.

https://www.chess.com/news/view/nakamura-adopts-komodo-on-fathers-day-20-5-2-5

Game analysis is by Stockfish 8.

If you are taking about the "play vs computer" with takebacks and hints,  there are obvious problems with those features and the games are not timed, so they can't be rated.

Instead of using the computer setting thing, use custom challenges and challenge Computer1 through Computer20. Those correspond to the 20 skill levels of Komodo 12. Hikaru lost to Lvl 20, although I suspect he could beat it fairly often. They are rated so you can get a feel how you are likely to stack up against various levels. Or do what I did. Start at Computer1 and work your way up until you lose.

Why stockfish 8 if stockfish 9 is available and better....

Why use Stockfish 9 when the development versions are available and much better...

Oh! I did not know that! Thanks for your info

To be honest, what engine runs behind the scenes makes no difference to end users like you and me. They can say they're running Komodo, but actually run Stockfish 6 in the background, and y you won't know the difference.

very true

Why does everything have to negate something?

Now, you really hurt my feelings. 

calm down everyone

Go to play chess (the hand on a piece symbol). "Live Chess" should be one of the options that then shows. Alternatively, click this link.

https://www.chess.com/live

Then select "Custom Challenge".

Type is "Standard" (the default)

Time is whatever you want. I like long play games so I select 45/45.

Rating is either Rated or Unrated. Choose what you prefer.

"vs" is the third and actually critical choice. Click the arrow to take you to the dialog for that. It should say "Play a Friend" and there should be a place where you enter a username. For the username, enter "Computer1", "Computer2", etc. up to "Computer20" to get the various reduced strength versions of Komodo.

They start easy and get progressively harder. To give you an idea, Computer1 is rated about 728 in rapid. Computer 4 is rated about 1153. Computer8 is rated about about 1597 at the moment. That is already better than about 95% of the players on this site.

*fare

how do you get the engine?

Honestly I was curious too because i mean it does affect how brilliancies are determined, right? a weaker engine might think highly of a simply "good move" or "best move" in the position while a stronger engine may think lowly of a brilliancy