General tools like an editor, a decompiler, or even an image editor can be used to modify the software itself. After all, if you're the creator of the tool itself, most likely you feel comfortable using it.
What I always wonder is how people built the first tools. How did people compile the first version of GCC? How did people build the first programming language? How did people built the first OS?
I can imagine ways to solve these, but I guess the tale behind how they did it is what makes me wonder.
Generally you start with "something smaller/easier" and build your way up.