Apple and Amazon will at minimum compromise your privacy to improve their products. And since they have no extra motivation since they don't make more or less money (because Siri and Alexa are loss leaders) they will have no extra considerations of privacy regardless.
Comparing Signal to protonmail is a much more interesting problem and you can go on to what has been subpoena from Signal and protonmail. Since there was one actually disclosed we can see the information (or really lack of) that was given by Signal [1] . We have a statement by proton mail on what can be subpoena [2] but there have been arguments against it.
[1] https://signal.org/bigbrother/cd-california-grand-jury/ [2] https://proton.me/legal/law-enforcement [3] https://protos.com/protonmail-hands-info-to-government-but-s...