App Stream authentication with AWS Managed AD

You should not need another SAML Identity Provider (ADFS), as you already have one (Identity Center) to work together with AppStream.

Follow the following documentation to setup the domain joined fleet: https://docs.aws.amazon.com/appstream2/latest/developerguide/active-directory-directory-setup.html

Follow the following documentation to setup Identity Center and assignment of the application to users/groups in Identity Center:
https://static.global.sso.amazonaws.com/app-eb7f4060188865be/instructions/index.htm (create an application, select AppStream 2.0 in Identity Center and then follow the instructions. this maps to Step 4 "Configure SAML 2.0" of the AppStream doc)