Source Code: Source code path is Project -> Assets -> Scripts. All source codes are in C sharp.
Some art work models, textures and materials are inside Project -> Assets -> ArtWorks, which includes building models, ground texture, grass texture, particle effect texture, GUI texture and other textures.
Prefabs are inside Project -> Assets -> Prefabs, those prefabs are repeatedly used in the scene, when you modify one prefab, related objects would be modified in scene. AI car model prefab, target prefab, road prefab, attach prefab and some effect prefabs such as sound effect and particle effect are inside this folder.
Sounds source file are inside Project -> Assets -> SFX, it includes car engine sound and car skidding sound.
Other files outside Project -> Assets is some settings and necessary files to correctly run Unity3D. Don't change anything.
To execute the project. You need Unity3D software environment. This project is designed under Unity 5.4.2f2 (64 bit). If you have Unity3D, then just double click Project -> Assets -> Project.unity to execute the project.