how do they do this u think?

Maybe it's not a mirror. They probably did a match cut to a different shot.

Also, that video sucked. I'm sure they could have gone somewhere with it to be interesting, but didn't.
 
It's obvious its a different shot because look how fast the transition is. The camera whips down then up. Its kinda obvious it was a different take because the mirror borders disappear then come back.

Motion tracking. AE. Two takes.
 
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