@Takahashi Ami • Please find my comments inline:
(1) Whether or not it is possible to create a registration form screen when actually registering as a member and customize input items.
Yes, this is possible. Below is an example of how registration form looks like and this document explains how to add additional fields in the registration form.
(2) Whether or not there is a mechanism to incorporate the input form set in (1) into the website.
Yes, the inputs by the users in above form can be returned as claims in the token. You can configure your code to read the claims in the token and display on the website/webapp. You can select attributes to collect and return as claims during creation of user flow or by editing existing user flows as documented here. Please find below snippet for your reference:
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