Are most GPIOs on common MCU's merely 'sourcing' or also 'blocking'?
"running through an output GPIO pin, back out of that pin" 

This is where confusion lies. The current cannot go into the pin, through some component inside the MCU, and then back out of the same pin.

GPIOs on most MCUs can be configured in the following ways:


floating input
input with pull up
input with pull down
push-pull output (actively drives line both high and low)
open collector output (actively drives line low only, requires external pullup to drive line high)