Rasperry Pi

pigpio

sudo apt install pigpio
sudo pigpiod

Ruby

https://github.com/nak1114/ruby-extension-pigpio

Also see https://github.com/parasquid/mcu-iot-sensors/wiki/Ruby#pigpiod

(you need ruby-dev)

custom hat to arduino shield form factor

SDA = A4 = 2
SCL = A5 = 3

RX = D0 = 15
TX = D1 = 14

D2 = 25
D3 = 27
D4 = 23
D5 = 22
D6 = 12
D7 = 26
D8 = 24
D9 = 13

CS = D10 = 8
MOSI = D11 = 10
MISO = D12 = 9
SCLK = D13 = 11

The Raspberry Pi has not ADC hence A0 to A3 are not connected directly to the pins. Rather, they will eventually be connected to a separate ADC module that likely communicates via SPI.