How did the Greeks manage to farm vast amounts of grain (vital to Athens) from the north coast of the Black Sea when these lands were in the territory of the Scythians?

Not sure exactly what your question is here...

Trying to break it up...

Greeks managed to farm vast amounts of grain because they had many colonies on the black sea each one with many farmers who were farming. What would happen is the city would know there was a shortage of a certain material, and so they'd consider their explorations up to that point and think about where those resources might be, and then organize the workforce to set up an 'emporium' which was a trading post and/or production facility.

Are you asking how come these colonies didn't get wiped out immediately by the locals?

Are you asking what specific fortifications were used to protect the colonies?

Are you asking what/why the Scythians might be totally cool with a colony in an area where they would occasionally ride through or past?