Now imagine how much Google (Gmail) knows about us. It gets scary very quickly. Even if you are as private as you can, you will communicate with people whose email is hosted by Google.
I think Google Search is much worse than Gmail. You tell it things you wouldn't tell your family or even your lawyer or therapist. And it can infer things you didn't even know about yourself (e.g. you are pregnant or suffering a health condition).
I had a recent exchange with Microsoft and a group of CISOs and how it was explained to US by MS is that Copilot relies on existing file sharing security (OneDrive, Sharepoint) to determine what user could receive as feedback from Copilot. While it seems like a reasonable approach to rely on existing controls it honestly sent shivers down my spine. Anyone who had some experience securing MS platforms data sharing knows those become a total mess overtime for large organizations.
I just have to look at my ancient msn account being effectively turned unusable for several (notably: Skype, Xbox) MS services due to being stuck in some limbo between MS auth service migrations (?) to gauge my confidence in their control of user data.... And no, several hours with their support agents spread over several weeks did not resolve it.
Their point is that that is wishful thinking. If someone violates the law, which does happen, your data can get out. "No longer yours" in this context means "others may disseminate it without your approval". They may be penalized for doing so, but it is absolutely in their power to do so.
Everything else is just wishful thinking. Like trying to keep a secret whilst only telling one or two friends.