iMessage Interoperability

Tim Hardwick, via MacRumors:

European lawmakers have provisionally agreed upon a new law that would force Apple to allow user access to third-party app stores and permit the sideloading of apps on iPhones and iPads, among other sweeping changes designed to make the digital sector fairer and more competitive.

[...]

Today's announcement focuses on services like WhatsApp, Facebook Messenger, and iMessage, which will have to "open up and interoperate with smaller messaging platforms, if they so request," according to the EU. "Users of small or big platforms would then be able to exchange messages, send files or make video calls across messaging apps, thus giving them more choice."

A few questions on interoperability between messaging platforms, from the point of view of an iMessage user:

• How is end-to-end encryption maintained between iMessage and the smaller messaging platforms?
• How do I know my details—phone number, email address, etc.—are managed appropriately by Apple (iMessage) and the smaller messaging platforms?
• Do I need to sign up to use the smaller messaging platforms before their messages appear in iMessage?
• Why does there need to be interoperability between messaging services when SMS exists?
• Can I disable third-party interoperability?

Sidenote: I don't want to see Microsoft Teams chats in iMessage.