The video covers the late Roman attempts to buy off the vast migrating herds of barbarians, and how it didn’t really work.
One very interesting source of Roman gold was from their invasion of Gaule, Britain, and other Druid influenced Celtic populations. The Druids were a priesthood of Celtic “intellectuals” who were responsible for divining the future. It required years of study (like a PhD) in occult matters and made them so powerful that they were exempt from Celtic military service.
They believed the spirit world was separated from the living by a large body of water and the way to influence them was to throw your valuables, including gold, into a lake. The Celts had amassed large holdings of gold that were initially stored in Druid temples and then cast into various lakes.
When the Romans invaded Celtic regions in Britain, they auctioned off the lakes to Roman speculators to raise money for their war chests. The Roman engineers would drain the lakes and often recovered large caches of Celtic gold and valuables.