Learn Live - Introduction to Azure AD B2C (youtube.com)
To be able to create new attributes, you need to add the Attribute Definition Administrator role to your user.
https://entra.microsoft.com/#view/Microsoft_AAD_IAM/CustomAttributesCatalogAttributeSetsBlade
Create Custom User Attribute in Azure AD – TheITBros
In Azure Active Directory B2C, custom policies are designed primarily to address complex scenarios. For most scenarios, we recommend that you use built-in user flows
The REST API technical profile allows you to send a complex JSON payload to an endpoint.
Define a RESTful technical profile in a custom policy - Azure AD B2C | Microsoft Learn
