Hi @kisappdev ,
This is the guidance to have a Meeting SDK operate as a meeting bot to join a meeting: Meeting Bots Accessing Media Streams
Please click that link again and you will see information about published vs. private (unpublished) apps to answer your questions.
Legacy apps were just the old UI and for app types. They were segmented into Meeting SDK, Chat App, OAuth 2.0, Server-to-Server, Webhook. Now, under unified build flow, you can enable these as different features under ONE APP containing the same credentials.