I’ve used it to play games from the Deck at native 1080p on my TV.
I’m not at my Steam Deck right now, but I remember it’s in the settings. I think if you go to the game’s settings, look for something like “native” display. You have to go into the settings for each game you want at a larger resolution on an external monitor in game mode and select “native”.
I don’t remember if it needs to first be enabled on the system settings in the display area. (I think it does the right thing for system settings by default in most cases.)
IIRC, desktop mode also automatically supports the native resolution, but game mode is nice and console-like. Desktop mode might be a bit clunkier than what you’d want for couch gaming. Setting the option in game mode for the game is likely your best option.