One must look Left and Right.
In fault conditions the amps are coming up the line ( the Poco ) and back from loads ( back-EMF )
The expression 'infinite bus' is a term of art. Obviously, even the entire US grid is not infinite. However, for the purposes of the calculations infinite bus means that the delivered voltage is not considered to sag. Yeah, it's idealized.
Because of the interposition of magnetic circuits within our infinite bus THEIR impedance becomes huge in short circuit calculations.
Ditto for 'rotary transformers' ie 3-phase induction motors and the like.
