Most games dangle some of the art assets in the open in the main file directory, such as "skins" which are often in JPEG texture file format, sounds in files like .wav, and effects in any one of various animation files in the model folders. It is not unusual for a game publisher to eventually release information on how to encrypt entire model folders, which allows 3D or 2D artists to create their own artworks, encode them, and simply drop them in to the object folder that is positioned in the main registry of files for the game. The object files are often labeled for the purpose of class files such as Maps, Vehicles, Characters, and so forth (really can be named anything by the game developer), but if you look at several popular game registries then you will notice industry standards which become obvious with experience (Maps being one common standard name).
Almost all publishers and/or developers have website forums which explain what you may or may not be able to do in the way of modding their game. Some are very restrictive but most allow at least some modding and publishing of your mods. A few are very generous in providing not only the knowledge on how to mod their game but also some of the game engine tools which they used to develop the game.
Research is the intellectual spine of game development. I actually learned much about game development by modding games over the years. Some modders are allowed by license to add or extend their own coding of the game, sometimes called "add-ons", so be sure to read the licenses if any exist for a particular game. You just might be surprised.