The most common practice nowadays is to build it with modular assets, and part of the games build process is to combine static meshes that share a material so it can be rendered more efficiently. This is a oversimplification of a complex topic, and there are far more advanced tools now. Most terrains aren’t even modelled anymore, but generated and sculpted in engine using PCG tooling.

Note, this is mostly for the contemporary AA and AAA games industry.