Market definition is a question of fact rather than law. So, in a jury trial, the jury decides what the relevant market is. (The judge instructs the jury how to make that determination.) The jury then determines whether the challenged restraint/activity generated anticompetitive harm in that market. (If not, the defendant wins.) If so, the jury determines whether that same restraint/activity also generated procompetitive benefits in that same relevant market. (If not, the plaintiff wins.) If so, the jury then determines whether there was some less restrictive alternative that could have achieved the same (or virtually the same) procompetitive benefits with no (or substantially less) anticompetitive harm, and, ultimately, the jury balances the anticompetitive effects against the procompetitive benefits to determine, on balance, whether the challenged restraint/activity harmed or benefited the competitive process.
The jury need not find the same relevant market as the plaintiff has proposed in order for the plaintiff to win but, as a practical matter, the jury will always or almost always come out for the defendant if it rejects the plaintiff’s proposed relevant market.