Who decides who wins?
I watch baseball because as far as I know it's not a complete setup. But with wrestling it's basically a live story?
Vince McMahon, whom is the promoter/CEO of WWE. The WWE has a staff of writers, some even from Hollywood, who write out storylines that go on for weeks on TV and culminate usually at PPV's which are once a month. Vince McMahon has the final say on if he likes the storyline/character and who gets to hold the championship titles.
A lot of times though some characters' popularity grows organically because of the fans. Some wrestlers who are trying to "get over", a term for having the audience draw any reaction negative/positive, end up becoming fan favorites whether good guy or bad, and it forces Vince McMahon's hand behind the scenes to say give that wrestler a "push" (push up the ladder to the top draw). Usually the fans can tell which wrestlers deserve their due or which one Vince is screwing and will try to get a certain wrestler over organically so Vince has no choice but to give him a title run.
As for matches, they're choreographed ahead of time. Before the matches, the wrestlers and the referee will discuss what's going to happen and what will take place. If you pay attention, WWE referees usually are wearing an earpiece, because they're getting direct instruction from Vince McMahon or his daughter backstage on how long the match is going or if something needs to change on the fly. The referee actually talks to the wrestlers very sneakily during the match to remind them that hey it's almost time to finish the match or don't forget to do so and so to get the crowd fired up.
And yes, wrestling is essentially a live story/soap opera. It's kind of evolved over the years. Back in the days of Bruno Sammartino, wrestling was treated like boxing. An actual event (because back then the entire secret that it was fake wasn't out yet. And you had bookies etc. treat it like real sports betting). Now that everyone knows it's fake, it's just basically a live concert/theater with theatrics, over the top characters, etc.