I really have a hard time helping you out; as it seems that you really do not have a grasp on security concepts as well as minimal experience with database access.
There is no quick and easy one size fits all answer. Programming is about knowing what the code you are adding into a project actually does and not copy and pasting blocks of code you find in various locations across the internet.
The same goes for asking questions on forums such as this as well as ASP.NET; at least put some effort into it instead of copy & pasting the same question across the web.
Probably the best thing for you to do is to build a completely separate project and then add in a pre-packaged identity provider to learn how it works and what needs to be done to implement it.
Introduction to Identity on ASP.NET Core | Microsoft Docs[
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After you get it working in your new and simple application; repeat the process.
Build another new application. Add the Identity Provided from scratch.
Document Everything!
Once you are comfortable with this; have a teammate create their own application, give them your documentation and identity provider package, an have them set it up.
Anything you are deploying should be documented and repeatable before you start adding into enterprise packages.
Once the above is worked out and done, then either you or your teammate can add it into the ERP project.