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Computers don't understand language. We covered this early on. They work with numbers, and nothing else. So before training can begin, all that text has to be converted into numbers.
Every sentence. Every word. Every punctuation mark. Each one gets assigned a numeric ID, and the whole training dataset becomes a very long sequence of those IDs. Billions of web pages, turned into a single river of numbers.
How exactly that conversion works, and what it means for the model, is a story worth telling in full. It comes later. For now, the important thing is just that it happens: text goes in, numbers come out, and the network trains on those numbers.
And the training task itself is simple.