Just look for any company that offers data annotation as a service, they seem happy to explain their process in detail[0]. There's even a link to a paper from OpenAI[1] and some news about the contractor count[2].

[0]: https://snorkel.ai/data-labeling/#Data-labeling-in-the-age-o...

[1]: https://cdn.openai.com/papers/Training_language_models_to_fo...

[2]: https://www.businessinsider.com/chatgpt-openai-contractor-la...