How do people in midnight club la complete edition get mods on their cars?

The reason i i'm asking this is because i have bought a couple of moded cars myself and i want to know how to do it myself.How do people in midnight club la complete edition get mods on their cars?
How to put them on? Not sure. How to get them? I think its all DLC, but people who buy the game now would have it already installed to their disk.