This sample should guide you
https://docs.microsoft.com/en-us/azure/active-directory-b2c/quickstart-web-app-dotnet
you get user data from token received from AADB2C and you can call MSGraph Apis
Yes, you can add a foreign key in your database which will point to user id in AADB2C to achieve this.
Server side would be more appropriate but depends on your use cases.
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