Normally, you wouldn’t be able to do that without extra parts, but on the ESP32, [BastelBaus] has found a nice hack — just connect your devices to different pins and slightly abuse the ESP32 ...

Plus the Arduino needs to be plugged into the host computer via USB to function, a rather quaint idea to many in 2018. These were just some of the reasons he decided to port Grbl to the ESP32 board.