Dragonborn live on the continent of Talon, a land ravished by dragonfire. Most kingdoms on Talon were ruled by Dragons, and the Dragonborn were their subjects. Depending on the kingdom, the Dragonborn were either citizens, worshippers, or slaves to their Dragon masters. However, over time, the Dragonborn overcame their overlords, and expelled them from power. The Dragons didn’t go easily, and burnt the land before they were driven away, leaving an arid, rocky wasteland in their wake.
The land was no longer able to support huge populations, and thousands died of starvation. Those who survived were the strongest, but also those who were most loyal to their clans. Now, centuries later, that strength produced clans that were based on strength and loyalty.
The clans are so important to the Dragonborn of Talon that religions are built up around Ancestor Worship, to the point that a cleric could call upon the powers of their clan’s ancestors instead of the gods. Most Dragonborn homes have a clan-shrine, but the larger cities on Talon have clan-temples, and in the grand city of Kvar, there is a grand cathedral holding a shrine to every single Dragonborn clan.
Typically, a Dragonborn’s core values are loyalty, and then determination. Everything else is secondary. To other races, this comes across as being stubborn, but Dragonborn respect others who hold their ground, in battles or opinions.
Same as in PHB (pg 32). However, intermarriage between clans in not unheard of. The offspring of such a union results in a mix of hues of scales. These mixed-clan children will have the breath of one of their parents (picked randomly), and are considered part of that clan. (For example, a red dragonborn male and a blue dragonborn female would have a purple child, who would either have their father or their mother’s breath. Whichever it is, that is the clan the child identifies with.)
This is not to say that each colour of dragonborn is a different clans. Many clans have multiple colours, with one exception: The Kaarev clan is always red. The more “pure” the colour of your scales, the higher your class. Too much mingling leads to a brownish hue of scales. The brownest Dragonborn are often outcasts.