I don't know why you're so obsessed with this? He just added a black Solid Color layer then masked it around the boy. It's kindergarten Photoshop.
He used the Shift key of course. https://www.damiensymonds.net/2010/10/shift-while-brushing.html
You're lucky. Every calibrator on the market will run on that computer.
Get the best one you can afford.
https://www.damiensymonds.net/which-monitor-calibrator-to-buy/
I don't think there should be a number there AT ALL.
Can you tell me WHY you're saving the photos as jpeg? Remember that jpegs must always have purpose.
Well you definitely didn't use 12 that time. It looks more like 0 or 1.
Go back to your master file, and save the jpeg again. Make very sure it's 12. See how it looks.