Federated = Email. Different email providers (Gmail, Microsoft, iCloud, etc.) can all communicate with each other freely and using the same protocol.
Defederated = No longer federated.
Not federated = Your company’s internal Teams/Slack/whatever messaging system. Even tho they theoretically could communicate without other messengers and/or people outside of the company, they typically don’t do they’re just their own little bubble.
Lemmy is federated, Facebook is not.
You should have a playlist already in your library called “Recently Added” and can have Siri or a shortcut play said playlist. I’m not sure how far back that playlist goes compared to what you’re wanting, so if you want to customize it more, you could setup a smart playlist to add all new songs to.