Bugs and symbols. There's a mac command line tool I can't remember the name of that pulls all the readable (and more) text from an application, this has been used to find out what made a closed source app (e.g. private iOS apis).
There are also tools which are platform specific. On macOS, for e.g., the Mach-O format can be disassembled by `otool`, for objective-c projects (and maybe Swift) you can use `class-dump`