Why does one draw solve the mislabeled boxes puzzle?

I’m prepping for a test and I keep getting tripped up by those “everything is mislabeled” logic puzzles. Say there are three boxes labeled Apples, Oranges, and Mixed, and I’m told all three labels are wrong. I’m allowed to pull a single fruit from one box. I keep thinking I need to sample from two boxes to be sure, but everyone says one draw is enough and my brain just… freezes. Which box should I draw from first, and how does that one draw let you fix all the labels with certainty? I feel like I’m overthinking a simple switch here, so a clear explanation would really help.