Most likely ground is single object and things you colored differently are a single entity. Textures most likely are reused throughout the city.
Ground and buildings might be handled in different fashion, though, which depends on their engine.
For example, they could have “City Block” object that acts as single entity, contains ALL unmovable objects (ground and buildings) merged into one entity AND determines what to feed into GPU depending on your position and precomputed occlusion data. Or they could use separate “static mesh” per every background object.
GTA 5 is not using Unreal engine, keep that in mind.