What is Epic Magic in D&D 5e?

I’ve been reading and writing some adventures for my level 18 party. Currently we are exploring Epic Magic. I’ve described the realm of Netheril, Karsus’s Folly, and Mystra’s Ban. I’ve explained that’s the reason they can now only cast spells up to level 9. I’ve described how Elven High Magic is still possible, and they will learn about it in the following adventures. But I don’t understand the concept of Epic Magic.

From random places online, you see things like

Part of the confusion came from the fact that previous to Mystra’s Ban mythals could be created by both high level (10+) spells and Epic magic. After Mystra’s Ban only the Epic magic option remained.

Or

Epic Spells – The following spells known in the Realms were created after Karsus’s Folly. Some were known by the same names as the Netherese high-level spells, or were recreations.

So I understand it is different from Elven High Magic or spells of level 10 or above. But what actually is Epic Magic? It comes from 3rd edition, and you can see stats for spells like Ioulaum’s Longevity or Diluvial Torrent. What does it mean for a spell to be Epic? Can it be learned by anyone? And casted? I’ve never played any other D&D edition except the 5th, so maybe this is a very obvious detail which I have never grasped before.