Hey there, the theme is present in that you're driving on the ground in a world that's moved on to flying cars and vertical cities. The dimension you lack is the 3rd dimension, in this case, up. The reason is because you are not of the upper class, I'm also playing with a social element thematically because you're trying to work your way "up" in the world, but you lack upward mobility, literally and figuratively. So, there's two dimensions to my Life in Two Dimensions take.
As for the game loop, I'm sorry it didn't click for you. I don't know where I lost you, so I'll just give you a quick overlook of the whole game loop. First, you leave the construction yard starting area. I bet you can figure out how to do that. Then you drive through a "Pizza Gate", you should see them, they're near the middle of the map, they are purple and have spinning metallic pizzas on top. You'll be assigned a destination to deliver your pizza, you can see it as a blue icon on your minimap in the lower right corner. Your car is the red icon so you can orient. You might also be able to make out a blue beam of light in the sky that rises above your target destination (it could be blocked by a building, but is usually visible). Drive to your destination which is represented by a big glowing blue box, get cash, repeat.
It might take a couple attempts to start making successful deliveries, don't be discouraged. Some destinations are quite tough, others are utterly simple. You may have just had bad luck.
That's the game! I hope that if you give it another shot, the gameplay clicks for you. Happy to answer any specific questions you may have.