This feature isn’t exclusive to the Leonoardo, as [Michael] explains in a build he sent in – the lowly Arduino Uno can also serve as a USB HID keyboard with just a firmware update.

[Damian] still needed input, output, and the ROM which made ... The 16u2 is normally used as the Arduino Uno’s USB interface. The only external hardware needed is a single 120 ohm resistor.

To achieve this, the USB bus is ... to the ESP32. “Uno R4 WiFi includes an error-catching mechanism that detects run-time crashes and provides detailed explanations and hints about the code line ...