When the pin is configured as an input, almost zero current flows in/out of the I/0 pin.
(The input impedance/resistance is around 100 megohms.)
Current is calculated as Voltage/Resistance (Ohms' Law
In your circuit, any significant current flowing through the pull-down resistor is flowing through the external
"Arduino Read" connection, not through the input pin. A 20K pull-up resistor pulled-up to 5V has 0.25mA (5/20,000 = 0.00025A) flowing from the power supply through the resistor... The current comes from the power supply, not the input pin.
If you connect a resistor to an output pin, then the current flows out of (or into) the I/0 pin, and you can use Ohm's law to calculate it.