One CL command that would be a good starting point is Work with Registration Information (WRKREGINF). This will show you the registered exit points on your system and, with option 8, what exit programs are currently registered.
To learn what the exit points do, a good starting point would be the API Finder. Under ‘Find by group’ is the option to show ‘All exit programs’. It really isn’t “All” as there are some other exit points in the system, but this will show you the primary ones.