A lot of times, Zoro uses his swords to cut down far away objects without any physical contact. How does that work?
I can't find screenshots but whenever you see blue things travelling from Zoro's swords, those are shockwaves that can cut anything, even buildings.
That's what he does, he sends a sort of shockwave. We obviously know that a human can't pull a sword (or another object) to create such a strong shockwave.
But if you think about it, if you move a wooden stick but towards the ground (as when you hit the ball in golf), you'll cause the air to move following that movement (adding various turbulences) that will affect whatever small is on the ground (e.g. sand).
Zoro's attacks are something similar to that but only much stronger. It's not realistic, but then, almost nothing on One Piece is realistic.