Old Valyria

Old Valyria is part of the Known World which includes a combination of magic and non-magic. It has the World of Earth that we know today combined with Westeros and other fantasy islands. This is the height of Valyrian culture.

Old Valyria gets its powers from the 14 flames they give the dragons that inhabit the peninsula powers and also the Valyrians themselves. This power allows them to ride the dragons and bond with them, and some are even impervious to fire. Other cultures wanted to experience the same gifts and pleasures as the Valyrians were, so the Vikings tried to invade their lands but ended up coexisting and now owning their own parts of Valyria.

The same with the Scottish people: they tried to raid in Old Valyria and instead their Queen Moira married the Valyrian's queen-to-be, Daenerys. Daenerys's father was Aegon the Conqueror. He gained more land in the Seven Kingdoms under his rule along with Old Valyria. Daenerys would inherit all of this, and she asked Queen Moira of the Scots to share it with her.

Then the Scottish people and the Vikings and the Valyrians all shared parts of Old Valyria and inhabited all of their cultures. That's why the flag has every part of the cultures that share Old Valyria.