As far as I know, Google and Apple are only used to send Push notifications to users. These are optional, and Signal on Android can be used without Google.

AWS S3 is/was used to share files with other users. It became a standardized API, with many alternatives supporting the protocol, so switching to a different provider or on-premises should not be difficult.

Signal intends to remove the dependency on phone numbers, and therefore Twilio, but this has not be done yet.

All IM apps rely on Google or Apple (on their respective platforms) for the push notifications. The alternative is polling in relatively small intervals, but that wakes up the app, which uses the battery without any real usage.

And if the OS decides to kill your app's process, your polling dies with it and you don't get notifications.