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

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.