Unless your playing one of the few games that used the Switch 1 IR sensor in the right Joycon, nothing.

Switch 1 games work “out of the box” on Switch 2.

IF the game you want to play just so happened to be one that needs the IR (or uses an accessory designed for the Switch 1 Joycon, you can connect a Switch 1 Joycon to the Switch 2.

Note.

Much like the Series X before it, many Switch 1 games perform better on Switch 2 even without a patch. Games that struggled, now hit their targets (Akrham Knight on S1 aimed for 30fps but barely got above 15fps, on Switch 2 it hits that 30fps target.)