When Petunia Dursley refused to take Harry in she forfeited his birthright protection, so Dumbledore took the baby to the safest place he knew: Hogwarts. The applicable staff (mostly just… not Snape) took Harry in on a rotating schedule as he grew from baby to toddler to child. They traded extra credit for babysitting among the older students, and Harry grew up knowing a few dozen different laps that were safe and warm to nap in. This was a Harry who grew up among books, among old transient walls and learned professors. They gave Binns night duty sometimes, and let him talk young Harry to sleep. This was a Harry whose world changed, on principle, daily. The stairs moved. The walls became doors. You had to keep your eyes open–you had to pay attention. So he did. He grew up in a school. Knowledge was power, but knowledge was also joy. This was his sanctuary. There was magic in his world from birth.