It's interesting to hear it from the point of view of the workers. History is usually tales of famous heroes. Which ignores... a whole lot The easiest good cypher I ever heard of is using an unknown language. E.g. Navajo code talkers.
No. A code is understood by those who know the code; apparently in the troubles in Bosnia 20 years ago, the British forces used Welsh speakers to communicate secret messages, since the chance of anybody there understanding Welsh were minimal.
A cypher is something not understood by whoever knows the cypher (until the message is deciphered).
The Enigma story is fascinating. There's quite a lot of read on Wikipedia about the deciphering efforts. What I find most astonishing about the "breaking Enigma" story is that one unfortunate decision (unfortunate to Wehrmacht, that is) - to type the message key twice - has given such a great advantage to people trying to decipher it.