Question: How is this a checkmate by opponent??

I'm black pieces, COM is white pieces. The final move was my queen to g3, but resulted in the COM winning the game. I thought moving my queen to g3 trapped the white king and would win me the game. Why did white win instead?

this is a stalemate you should try to mate by having the queen touch the opponent king while simultaneously touching your king against the backrank next time

It is not a win by white, it is a draw because of stalemate.

Here is the definition of stalemate:

https://support.chess.com/article/682-what-is-stalemate

You mean the computer swindled you of your rightful win. That's not equivalent to the computer winning the game.

(The stalemated side does not win a game under the current rules)

This position is a Draw. This is due to a rule in chess known as "Stalemate".

Here, the king is not currently in check, but can not move anywhere because all the squares are occupied by the queen, and the king does not have any alternative moves since the only pawn he has on the board is blocked by yours.

Because neither the king or pawn can move anywhere and the other king isn't in check, that's a draw by stalemate.

The question was how and why white won.

white didn't actually win 

Check out the drills - they have this situation so you can practice finishing with a queen. I've done this before - that's why I needed the practice!

White didn't win it was a stalemate

He had no where to move and wasn't in check, so it resulted in stalemate and draw

If the result of the game written on the screen is "draw by stalemate", and the guy reads it as "white wins by checkmate", then he has much more serious problems than just not knowing the rules.

I'm afraid it was a draw by stalemate