Based on your username in guessing you're German? (Apologies if I'm wrong). Here in the USA when I first got my license in the state of Oregon it was the opposite. If you weren't looking backwards while reversing and using only your mirrors, you'd be docked points on your exam.

Smaller cars I would definitely look back, because you get a wider view, but it's also situational. If you need to quickly alternate between left and right side I would use mirrors, especially if squeezing in somewhere.

Especially with an SUV like we have now, looking back does nothing. Small rear window, all I see is back seats. Now I use mirrors and top-view mode + rear view on reversing cameras that surround the vehicle.

Probably just a shoulder check used for photo. Probably an unrelated stock image for the transmission. Still a funny meme template