Are 3D video games really 3D

Once all this is done, the assets are typically processed by the Toolchain which turns the 3D Modeler friendly files into game ready data which can be loaded directly into the Runtime.

Toolchains (the bit that takes the raw assets and transforms them into game ready data) are often some of the most complicated pieces of engineering in game development. They need to talk to many different applications, understand many different file formats, and also understand how to transform data into a more efficient and instantly usable format. Toolchains typically also perform lighting pre-passes, vertex welding, LOD generation and so on. Often building the entire data set for a large AAA game will take a very long time, often greater than 8 hours for a complete rebuild. So Toolchains typically also contain distributed processing to spread the load across all the PC's in the entire studio.

The Runtime is the game itself. The executable which runs on your PC, or Console. Inside the Runtime there will be a system called a Resource Manager. It's the Resource Manager's job to load the assets requested by other systems within the game.

E.g.: When you tell the game load into a Level, the Level's metadata will provide the Resource Manager with a list of which assets need to be loaded to display the level. Likewise, each Asset will contain a list of any Textures, Materials, or Sub Assets that are required to be loaded.