Let's say you have a corpus of text — 10 million words — about a specific topic.

1. What's the best way to "train a model" on that text?

2. Is that even the right term? Or is it using an existing foundational model and then augmenting it? Fine-tuning it? Something else?

you are describing fine-tuning

OpenAI custom GPTs make this very easy but I’m not sure about the max upload size or what type of interface you’re looking to use for your specialized model

fine-tuning a Llama 3 is probably the best bet if you want more than a quick hacky solution