If you use Signal, Discord, or any other messaging app and you DON'T want Google or Apple monitoring/reading/learning from your messages, follow these steps.

Android:
1. Open Google app
2. Tap your profile photo
3. Settings
4. Google Assistant
5. "Your Apps"
6. Choose the app (e.g., Signal)
7. Toggle "Let your assistant learn from this app" off

iPhone:
1. Settings
2. Apps
3. Choose the app (e.g., Signal)
4. Toggle Apple intelligence or Siri settings to off (“learn from this app”)

Note that your Android instructions may differ depending on your model. Those worked for my Pixel, but please feel free to chime in with directions for other models.
@EnfysBook I strongly recommend simply disabling the Google app altogether. At least that was possible on my phone. In fact, I've disabled them all except the Play Store and Play Store Services. But only because I got tons of errors when I tried disabling those. 😅
@veronica @EnfysBook I hope you appreciate how much absolute evil permeates the services library. It *is* the big back door.