@inthehands I’m hoping someone can provide more technical details, but this is a good summary of my layperson understanding (that on Apple, these “learnings” are on-device or synced encrypted between your devices): https://www.intego.com/mac-security-blog/no-siris-learn-from-this-app-setting-is-not-sending-data-from-your-apps-to-third-parties/
I believe that, today, each app can define what data it exposes to this system, so an app like Signal can decline to provide message contents. (That may be an incorrect understanding.)
If you disable this setting, the app itself won’t even show up in search.