The largest source by volume is the one frozen inside the model itself. Before ChatGPT, Claude, or Gemini ever answered a question for a user, they were trained on a massive snapshot of public text. Books, websites, articles, forums, code, scientific papers. The snapshot for the most current models is measured in trillions of tokens.
That snapshot has a knowledge cutoff. The model knows the world up to a date and stops. Cutoffs typically sit months to over a year behind today. After cutoff, the only way new information enters the model is through the next training round.
If your name lives inside the training corpus, the model recognizes you without needing to look anything up. It can describe you, attribute claims to you, and recommend you for prompts that match your topic, all from internal weights. This is the highest-confidence form of citation. It also takes the longest to earn.
What actually ends up in training data is the kind of source that survives crawling and curation: Wikipedia entries, established media articles, podcast transcripts on major platforms, books indexed by Google, GitHub READMEs, Reddit threads above a certain karma threshold, technical documentation. Random LinkedIn posts and ephemeral marketing pages mostly do not.
The leverage in this source is patient and structural. Get into the kinds of places future training rounds will pull from. The work compounds across every model release.