The Miyoo Mini + has a USB-C port but its only goal is to charge the battery. This hardware mod will let you use your USB-C port to plug any USB device to your console (joysticks, keyboard, mouse, bluetooth dongle etc.)
As we can see in the datasheet, pins 111 and 112 are respectively DM_P2 and DP_P2 (second port's USB differential pair).
Below, we can see a the footprint of the SoC on the PCB. This second port is not used, no track nor vias are visible.
Below an example of the result:
The Miyoo USB-C port is only used to charge the battery. In other words, nothing inside the console will generate a 5V on your USB-C port.
My quick & dirty temporary solution is to use an external 5V power supply which is my USB-to-UART dongle.
I don't want to charge the battery of the console with this external power supply so I'm using a USB-C male to female PCB on which I cut the VBUS trace and plug the external 5V power supply on the female connector side. (you can buy one here)
I'll soon share retroarch configuration files to use joysticks and enable multiplayer on your Miyoo Mini + !