"OAuth 2.0 is a specification outlined in RFC 6749 that allows third-party services to make requests on behalf of a user without accessing passwords and other sensitive information. If you are unfamiliar with OAuth 2.0, check out Aaron Parecki's "OAuth 2 Simplified"."
SFSafariViewController can be used to display web content within your app. It shares cookies and other website data with Safari, and has many of Safari's great features, such as Safari AutoFill and Safari Reader. Unlike Safari itself, the SFSafariViewController UI is tailored for displaying a single page, featuring a Done button that takes users back to where they were in your app.
OAuth provider guide