Once you have all that, you wait at the gate until your flight is called for boarding. You have to check one of your carry-on bags. You don't have a seat. Your flight is delayed or canceled.
Airlines limit the number and size of carry-ons.
If you are making connecting flights, one plane's configuration may be different from another's. You also pack in your carry-on whatever clothing you may need during the first 24 hours at your destination. The way things are, you're lucky to have a seat at all. Airlines overbook seats, and it's legal. For the rest of the huddled masses, the airlines certainly accommodate requests if they can, but make no promises.