Look at the A/B terminal map from the airport website
https://www.orlandoairports.net/getting-around-mco/. The original terminal building is divided into an A side and a B side. When you get off the plane, you’ll be at an”airside” mini-terminal and will have to take a little train to the main building. Depending on which airline you fly, you’ll arrive on either side, with separate baggage claim on each side. The rental car agencies also have locations on both sides. When you reserve a car with them, they want to know your arriving flight information so they can have your car on the correct side based on which airline you’re using.
There is now a terminal C completely separate from A/B, and the rental car agencies have locations there, too.