To clarify things, IAS does authentication, AMS does authorization, XSUAA does both authentication and authorization, which will eventually be replaced by IAS+AMS.

Currently, SAP Cloud SDK Java has a beta support for using IAS. (JS does not support IAS and is aware of this feature request.)

AMS has its own library. And assuming you are a CAP Java user, it is already feasible to integrate a CAP application with AMS. Check https://cap.cloud.sap/docs/releases/oct24#tool-support-for-ams for tool support when using AMS in CAP project.