Each car exists as a unique subfolder in [content]\cars and is automatically recognized by the game if it holds the file "carinfo.cca" inside (more on that later). Other files such as 3D model, shop data, engine sounds, textures and default / custom wheels can be found in models, shop, enginesounds, textures and wheels respectively.
To share your custom cars with others, use the Workshop Tool to upload your car to the Workshop, provided that you exported everything correctly into a cpk archive. To install custom cars, simply subscribe to a mod listed under "car" in the Workshop.
If you installed or created a car correctly, it should appear available in the game's career shop.