Usually a 3D artist would retopologize the model in a tool like ZBrush. For some models they are automatically "decimated" where the model is simplified using a

Usually a 3D artist would retopologize the model in a tool like ZBrush. For some models they are automatically "decimated" where the model is simplified using an algorithm. Decimating is not that common though in high end studios though. With Unreal Engine 5 a feature called Nanite is supposed to allow high polycount models to be handled without preprocessing using some kind of runtime algorithm that simplified geometry. These models also need all the lighting information extracted and turned into textures for use in a game engine(albedo, metalness, roughness etc). Most tools for automatically extracting the lighting information get it wrong most of the time leaving the artist to recreate it with a tool like Photoshop.