Only the year is kept. The day and month are used to check you are 18 and then discarded. It cannot be changed afterwards without contacting support.
Listening, following and saving, but no uploading. Ferality is invite-only for now, and one code opens either door, so pick by what you came to do rather than by the code you were sent. Nothing is lost by choosing this one: a listening account can be given creator capabilities later without signing up again. What's stored about listeners ↗
Uploading audio and writing scripts. You'll read and accept the creator agreement on the next screen, before the account is made.