Sitecore Identity (SI), introduced in Sitecore 9.1, is the single sign-on mechanism for any Sitecore instance (XM, XP, XC, …) that requires authentication.

As it was provided in a web deploy package (WDP), making small config changes like integrating with Azure AD was straightforward. With Sitecore moving to containers, and SI thus being provided as container image, making these same config changes becomes more complex… unless you’re willing to throw some Kubernetes features at it.