I hate the design, but that's really well done. Angled tile going around a corner? That's some challenging work! I don't even know how you'd cut those tiles, and you can't bend tile.
Good callout. The tiles are cut halfway, stiching them together with white grout/silcon along the edge so it doesn't stand out with the normal grout. At least that'd be my guess.
I honestly can't see a way that doesn't result in some visible something (although maybe not visible in a photo). You could throw money at it and get custom tiles made, but you'd have to have your measurements and tiling technique perfected to make that work.
I can't tell if the mirror is flush with the tile or if it was just outlined in grout, but either way, this contractor cares about the details. I would've just slapped the mirror on top.
Edit: Someone help me out. Is that sink really small, or are the tiles on the top sides of the sink extra long as part of the illusion? Are the white tiles on the left wall square?