How the hell are there already shirts, hats and even jerseys made for the winning teams of some championships?

I've always wondered this. Like, are the people predicting hard as to who'll win? And how the hell do they make all of the shirts and hats that the winning team will celebrate by wearing just in time that they won the big trophy by?

Have other teams gotten things made for them once they got far in like the NBA Finals, the World Series or so but when they lost, they had to discard the made hats and shirts?

How does any of this work?