The SDL library is modular and only the core is distributed, by default, when you acquire the library. For SDL 1, this includes SDL, itself, and (for those writing SDL applications) SDL-devel; for SDL 2, the libraries are SDL2 and SDL2-devel. The corresponding include files in developer's applications are <SDL/SDL.h> and <SDL2/SDL.h>, with the include files having been installed in whatever location is standard for your system, when the *-devel libraries are acquired and installed.
The modules all follow a similar pattern, SDL_X, SDL_X-devel for SDL 1 and SDL2_X and SDL2_X-devel for SDL 2, for module X, with the corresponding developers' include files being <SDL/SDL_X.h> and <SDL2/SDL_X.h>. For instance, for the image module, X = image, the libraries are SDL_image, SDL_image-devel, SDL2_image, SDL2_image-devel, and the include files are <SDL/SDL_image.h> and <SDL2/SDL_image.h>.
The modules are: "image" (as just mentioned) for handling images in the different standard formats (e.g. PNG); "mixer", for handling the different audio file formats, "gfx" for graphics drawing primitives, "net" for networked applications, "rtf" for Rich Text Format, "ttf" for the font-handling (ttf stands for "TrueTypeFont").
The Source Code for SDL is over here:
https://github.com/orgs/libsdl-org/repositories?type=all