I didn't say "the next version of Stockfish".

You have Stockfish version n, see board state s. I have Stockfish version n, see board state s. I want to know what you're about to do, so I put Stockfish into state s, ask it what the best move is, and I know you'll make that move because I know you'll ask Stockfish version n the same question of the same state. I now know board state s+1.

The steps are not pre-programmed, but the program itself is (modulo hardware imprecision) deterministic. If there's a RNG in there then sure, this doesn't work as easily as I wrote it; and there may be randomness in the thing that this is a metaphor for, regardless of if there's one in Stockfish or not, but that's not hard to work with when you want to win against an aggregate: we invented the field of statistics to deal with random numbers because they come up so often.

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.