I found out earlier that Undertale can be played with a keyboard on the Nintendo Switch, and then I tried whether it works with other games, but it didn't work, somehow only with Undertale or can you play other games on the play switch with the keyboard? An answer would be nice.
I never tested it myself, but I just guess that you can only play games that support it. For example, Mario kart, Zelda, Splatoon, … Will not support it because consoles are exclusive games. Undertale, on the other hand, is also available on the PC and thus supports mouse and keyboard. You could try it at Witcher, Payday, Gangbeast, … The main thing is that they are not Nintendo games because they can never be played on a PC.
Buy USB C to USB adapter, and then connect the keyboard.
Yes, you can also play Zelda etc. With a mouse and keyboard, but you need sun adapters for this.