The video below shows how to connect another Arduino to do the programming. You could also use any dedicated AVR programmer you happen to have.

Using a small OLED screen in combination with the Arduino Nano together with a couple of buttons and 16-bit ADC for precise measurements the multimeter has been finished within simple 3D printed ...

The Arduino Nano RP2040 Connect board is fully compatible with the Arduino Cloud and because the Nano RP2040 Connect is a connected device, this opens up all kinds of possibilities.