The quick answer is that the Enigma machine (like every other modern cypher method) used a cypher key. Knowing how it worked was an important part of breaking the code, and the Poles who got ahold of one are credited with that, but the hard part was getting the new key every day, a task that more or less required inventing the electronic digital computer from scratch.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams