There is deliberate randomness in stockfish. The easiest way to see this is from the fact that, when playing the white pieces, it won't play the same opening move every time. Often it's e5, but it also goes for e4 or Nf3 or something else entirely.

This is by design, and very much necessary for a competitive chess engine. Otherwise, people could do basically what you say: Run an offline (as in, ahead of time, with ample compute resources) search against stockfish that finds a line where it loses, then make an engine that plays that every time.

As a consequence, even if you know that your opponent is running stockfish, you can't really use that against them. Your best bet is also just running stockfish.