I now found the documentation about the pins (https://docs.micropython.org/en/latest/esp8266/tutorial/pins.html) and was able to trigger a callback when pressing the big M5 button (btna):

#Define Callback: def on_wasPressed(p): print("Button was pressed: ", p) B_M5 = Pin(37, Pin.IN, handler=on_wasPressed, trigger=Pin.IRQ_FALLING, debounce= 500)