The steps described in this article apply to App registrations (legacy)
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Actually it looks like only the Azure AD v1 steps apply to App registration (legacy). I'm not sure what the Azure AD v2 steps refer to, but it's not the current design.

There is no "Add an App," no "Application Secrets," no "Generate Password," no "Platforms," and no "Add Platform"

Looks like Azure updated their interface. I'll review and update the article.
Quick update. A draft of the updated article is in review and should go live in a couple days. I'll close this issue when the associated PR is pushed.
The updated article is live. Please open a new issue if the new article has any flaws.
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Looks like Azure updated their interface. I'll review and update the article.