You could literally read the GraphineOS documentation whenever you want where it clearly outlines how their hardening works, what security systems they have in place, and how their encryption works. I will warn you its incredibly technical, of course you wouldn't know that because people like you who cant bother to even look at the website (https://www.grapheneos.org/features) wouldn't know what any of it acturally means.

Tldr: for people who dont understand technical terminology they can trust the project, for people who do they can literally just read the documentation