There are two main ways they accomplish this. The first is by downloading low resolution textures first. Games use low-res textures when you are far away from an object and use higher res textures as you get closer, which get swapped out in real time. By utilizing this system that is already in the game, they can simply not load the higher res texture if you don’t have it downloaded yet.

The second way is by not downloading assets that are only used in other maps. It can download the core assets for the game and not worry about assets that are used on maps you aren’t in. However, if you do enter one of those areas before the content there is downloaded, they need to add a system to either download that area’s assets first or tell you to run the downloader longer.