You lob it the beginning of a document and let it toss back the rest.

That's all that the LLM itself does at the end of the day.

All the post-training to bias results, routing to different models, tool calling for command execution and text insertion, injected "system prompts" to shape user experience, etc are all just layers built on top of the "magic" of text completion.

And if your question was more practical: where made available, you get access to that underlying layer via an API or through a self-hosted model, making use of it with your own code or with a third-party site/software product.