You just follow it to chord progression. Translate it to the instrument. The lyrics come naturally. Maybe it's easy for a man who moves freely from piano to guitar to harmonica. Maybe it's genetic.
Any art is like that.
Not that he hasn't had to work for it. Bramhall spent a lot of years working days in business management, chasing his music at night. You underwrite your own profession. And you master your craft. You can correct a lot of faults if you work at it.