The general way to do it is to create a collection to store the per-user data.
one of the fields will be the user’s ID (or email if you prefer).
using wix-users you can get the current user’s details and use them to insert stuff to the collection.
an interesting point to consider is permissions - who is allowed to set user data.
if you provide a bit more info on the use case we can discuss how to do it right.
good luck!