A standardized signups component could help apps that put low effort on this step, but I don't know how is possible without a webview.

However this could not be a rule, it would kill creativity and personalization on new solutions.

An onboarding app with a good educational support can work well for non tech users. And it implicitly and practically shows the concept of an open ecosystem. A lot of users are fashinated when they see they can move from app to app with the same account, having a continuous experience.