There's also the difference between building bottom up with full knowledge of what you built, like the Apollo program team had (even if they bought something externally, the people who made it were available to the team, and/or the team studied the items in depth).

And then there's modern world of slapping together libraries that are often not known, sometimes with closed source, sometimes without real documentation. I've had to use disassembler in the past to access framework we were supposed to use...