I've reached the saving part of my game, as in, you press save and it writes to a text file, but I've noticed that most games and programs out there use custom save formats, like Dwarf Fortress DAT files. What are these? are these just text files formatted for the program, or are they a lower level thing?
Many games use a proprietary format for saving data, which they serialize in either plain text or binary form. The format of that data is entirely up to the game, as is the extension the author chooses to use (if one is chosen at all).
That said, all the extensions you've listed probably aren't saved game data at all, because they all have canonical uses elsewhere. PPT and PPTX are usually PointPoint slide decks, JPG and JPEG are image files, and KEY, while less of a de-facto standard, sounds more like a file used to store license key information or something rather than game data.