Upload Data is where you would send inference data to Arize - there's an option to upload data via CSV, among many other options. Docs here
Datasets is a subset of your data curated for evaluation and experiments -- e.g. if there are specific LLM spans such as ones that indicate user frustration you want to save to test prompt changes. Docs here.
At the moment, there isn't an option to load examples to a dataset via CSV, but curious to learn more about your specific use case to share with the Product team.