ABC is written in MS Visual Basic 6 (or Classic VB as it is now called, since MS brought out Visual Basic NET -- and yes, we are also working to rewrite ABC in NET but this may take a few years). It also uses a few libraries of functions purchased from other companies -- the Excel compatible spreadsheet is an example. ABC Source Code.
How do we find our way around all this stuff? Well the internet (since 1980) has dramatically changed the way we (and I guess, all programmers) work. We swap information and send tasks to people anywhere in the world instantly. Support Forums answer our stupid questions (fortunately we can remain anonymous....) and in return we help others with odd programming problems. Bits of ABC were written by people in Australia, America, Holland, Germany, India, and of course Lancashire. Most of these workers we've never met.
These days most software tools do not come with any printed documentation -- it's just a file on a CD. I suppose we understand this paperless system, but often the first thing we do after buying some new product is to get onto Amazon and buy a few books. The reason we like Amazon is that you can read genuine user reviews of the items to help decide what to buy. (Incidentally, ABC is now available on Amazon).
After a while the bookshelves are full (and threatening to fall off the wall) so we collect the old books and take them to a local charity shop. Here is a picture of Chris Hicks cowering behind some books.