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Don't ask why BUT after all this time, I realized this guy using a "Loupe" tool throughout his tutorial. Is there such a tool/plugin within Photoshop or is this most likely done in post? I hate asking such question but it's like a bad itch. ;)
I'm pretty sure that is a third party Mac app called Zoom it.
I thought I'd also point out that this is a built in thing in both Mac and Windows. Just a bit different looking.
Preferences > Accessibility > Zoom
Settings > Ease of Access > Magnifier
Required, but never shown