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

Yeah the most secure android operating system vs a bunch of custom roms which are significantly less secure than stock due to less updates, being less hardened, and using insecure components (like reverse engineered gapps)