Powered by an ATMega328p pulled out of an Arduino Nano ... A basic user interface is provided by two tactile buttons on the PCB as well as an 128×32 I2C OLED display. In a future version ...

But we can’t deny this Arduino calculator ... For the user interface, [Danko] is using an array of 17 tactile switches for the keyboard and a very crisp 128×32 I2C OLED display.