When you add the Wix Members app to your site it will set up a new collection in your dataset area in your site structure on the left hand side of your Wix Editor. It will be listed under the Members heading.
This collection can’t be added to as it is read only and will only show the collection data to each specific user only, so a site member will only be able to see their own data.
If you want to do a custom form, then you are still better off doing it through the seperate Wix members profile tutorial.
https://support.wix.com/en/article/corvid-tutorial-building-your-own-members-area
Remember that you don’t have to keep it as a profile page, you can change the user inputs to whatever you want to put on the page and design it to how you want it to be setup etc.