Online multi-player works as it would for any PC / Console.
For local / shared / split screen games we have some additional Xbox controllers we pair via Bluetooth to the steam deck then connect it to a TV via HDMI and we all play with the wireless controllers.
Party games like the https://www.jackboxgames.com/ are even easier, you just hook the HDMI up to the TV or stream it to the TV using Steam Link then everyone uses their phones as controllers.