Do they model the whole massive game level and apply boundaries where when the player jet go outside them, they U turn the plane or do they just keep duplicating the terrain far in front and all sides of the player and randomize it giving the illusion of never ending terrain?
The earth (game terrain) is truly flat on jet combat sims because it ends.
I’m not familiar with the game, but a quick search shows the engine used is UE4.
There was a GDC talk a few years back where they discussed the then new open world tech in UE4. If you happen to have UE4, you can also download the demo shown in the video. It is a pretty massive map.
Usually these type of terrains are procedural y generated, meaning there are no models and other 3d assets imported. They are generated by code.
They can be randomly generated too but usually that happens one time and it stores the level so you wont see a different map for the same area each time you play, unless that is the goal.
Streaming and using LOD assets can help too to drop polycount. But this is something you can do with static maps (traditional approach) or proceduraly generated ones.
Baking also can play a huge role here, as can do associate things like normal maps etc. Generally speaking its very easy to fool the human eye into believing on what it seeing is high in detail while in the reality its not. Unreal makes heavy usage of baking for lighting which frees a lot of GPU power to accommodate for higher polycount.
None of those techniques are new, 17 years ago Morrowing proved the unthinkable at the time by removing the loading screen completely , something we take for granted nowadays. GPUs back then were extremely limited and games had to rely heavily on CPU. So 2 decades afterwards its not surprise that such technologies are taken for granted.
fustrum culling helps in big maps
Lots of tricks can be used, including procedural generation and “tiling.” As you fly past one area, presumably never to return, the game engine can “harvest” those resources and re-apply them to the areas you’re about to visit, generating the terroir as they go.
You might actually notice this in some games where, if you turn around and fly the other way, the terrain has changed. (But, most games are “chasing you” with bad-guys and good-guys of various sorts, so you rarely have the opportunity to do that.)
Even a very big “terrain” can be represented using very little data, and that representation doesn’t have to be an (x,y) grid – which is wasteful. If the GPS in your car can contain an offline street-level map of an entire nation, a vast game terrain can be represented using analogous techniques.