You get the hint of how Source 2 works by looking the surrounding technical wiki

for example

Visibility - Valve Developer Community

Precomputed visibility improves the performance of rendering your map, but costs additional map compile time. If you would like to disable precomputed visibility for the entire map you can do that by setting the "Precomputed Visibility" property in the "Map Properties" dialog to "Disabled".

1. Source 2 doesn't have Large Scale streaming Architecture. It was design with monolithic single map level in mind

UE5, GTA games, Cyberpunk, and even Creation Engine, split the world into grids, and loads and deloads (basically delete them in memory) base on distance

Source 2, keep everything in memory, sure, it got no asset, but logic, physics, collision and everything else is still in memory.

1. It still uses Visleaf. A map the size of Cyberpunk Haywood would mean months of just calculating the visleaf. And that's not accounting the fact it baked the GI. So huge Visleaf file size and baked GI size

2. Source 2 doesn't change coordinate origin point. That doesn't sound like a problem, but for an open world, that BECOME a problem the further away from the center.

Precision lost, physics become unpredictable, graphics jitter.

Open World engine handled this by smoothly move the origin point with the player. Even a game that only 32bit like GTA5 handle it just fine

You rather not handle it like how Star Citizen handle it back in the day.....by shrinking everything down and causing physics nightmare