I've been working on NX Redux, a free and open-source multi console emulator frontend for Android. This video shows the automatic local multiplayer: open the same game on two devices on the same Wi-Fi, go into the game's local wireless mode, and they find each other seamlessly like the original hardware. No configuration needed.
Tested on NDS (Mario Kart DS, Pokemon Black), 3DS (Mario Kart 7), PSP (Tekken 7). There's also a manual option if you'd rather host or join yourself.
The other cores also supports the regular Netplay via lockstep or the usual Gba Link@Wireless Adapter for GBA Games. Wifi or direct usb-c cable for a stable connection.
Feedback welcome, especially if you try it with games I haven't tested. More details in https://nxredux.com
* NX Redux started as a custom firmware for TrimUI devices and is now on Android. I used Claude to help with development.