I do not understand the ending of this tactic

en.lichess.org/training/1186

I found the move Qe6 readily, but it marked me incorrect when I played Qxd5.

In my mind, why would I want to keep the queens o the board? I am going a minor piece up in an endgame, and you retake the queen with tempo, which gives you enough time to snag the c3 pawn. I played it out with stockfish and won

http://en.lichess.org/kdCXbN7Il8ZR

so I do not see the advantage of keeping the queens on the board

Qxd5+ is at least as strong as Bxd5+. Just an alternative that slipped through the cracks.

Your understanding is fine :)