Still, to be fair to the video here, considering Brandon the Builder was the founder of House Stark and his reputation as a craftsman, wouldn’t that suggest he was technically of small-folk origin by birth who capitalised on his engineering prowess/feats to become the King of Winter, thus likely starting off as someone’s subject/vassal before climbing the social ladder after the Long Night?

I doubt it, for a couple reasons: 

1. There’s at least some legends that suggest that Brandon the Builder was a grandson of Garth Greenhand, which if true would make him highborn indeed. 
2. Bran would have had to have been highborn in order to give the Gift to the Night’s Watch. 
3. Given the high degree of likelihood that Brandon the Builder was the Last Hero, the fact that he started out with “a sword, a horse, a dog, and a dozen companions,” suggests noble status.