What LLM is used to convert text to

Not an LLM, it's an embeddings model 🙂

By default, it is text-ada-002 from OpenAI

i saw gpt-3.5-turbo being specified in the example of make SimpleVectorIndex, thus it will call gpt3.5 rather than Curie002?

Only for the LLM model, embeddings are specified separately 👍