Imagine Reddit as one large city. It contains several neighborhoods that are subreddits. You can move freely between these neighborhoods, but they're all ultimately under the control of the mayor.
Lemmy is meant to look and feel similar, but we have several cities with several different mayors. Don't worry - the mayors have an agreement so that you can visit neighborhoods from all cities freely. But if one city burns down... The others remain up. And if one mayor becomes crazy, the other cities can ignore them.
Apart from that, your experience should be similar. Find communities you like, follow them, post, comment. Just keep in mind it's early days so the apps and website still have a few bugs here and there, they'll improve with time.
Does this have a polarizing effect over time? Is there a "rational" part of Lemmy and an "irrational" part that mutually block each other's cities? (Superman and Bizarro Superman if you prefer that analogy)
When a city (instance) gets too populated with users. Can the mayor stop people from coming to our city and redirecting them to cities that still have capacity?
I dont want the city to get so big the mayor can't handle it.
Not only they can - this already happened. Lemmy.ml locked down new account registrations as they couldn't keep up with the demand, which accelerated the growth of Lemmy.world.