From what I've read / watched, Rage 8, the engine used for Red Dead Redemption 2, was basically a prototype for doing GTA6, which is on the Rage 9 engine.

These large open world games want you to think there is a large, city scale sim running, but before RDR2, this was all just smoke and mirrors at best.

One key thing Rockstar has been working for GTA6 is separating the various simulations from each other, so that local car and player physics, general traffic, pedestrian pathing, etc, are all fully isolated, and in fact will run on different cores of the processor.

The end result is that you can have large world scale simulations of traffic and pedestrians running all the time, without wasting any graphical or physics processing power on them. That's why the world of RDR2 feels like it's happening whether your character is there to experience it or not. Unlike most open world games where you can just tell "I'm the main character and the world is just a local bubble projected around me".

Even in GTAV, which is basically the best of the old way, you can still see vehicles spawn in and out of existence. Cops just appear on the map because you have a wanted level, things feel like west world, where the animatronics only start their scripts when you walk in the bar.

GTA6 is supposed to do away with all of that. Cars, people, animals, the weather, will persist even when you aren't there to look at them. Events will just be going on in the world. Pedestrians will report your description and car to the police, which will then dispatch local cops to chase you. These various systems able to interact with each other is going to provide far deeper emergent game play across the board.

Far more than higher poly count / more pixels / bigger map, having these core game systems isolated from each other and able to run continuously, will change what a video game world is and can feel like.

When you asked if people were really excited about the game, this for me is at the heart of the hype.