Graphene is a hardened Android OS based on AOSP.
Calyx is also based on AOSP but without most of the security benefits and extra hardening. With their often delayed security updates I would even entertain the point, that Calyx decreases security compared to stock.
Granted, there is less tracking going on on Calyx vs Stock Android.
There is much nuance to this so it's best to read up on it. Start here:
https://blog.privacyguides.org/2022/04/21/grapheneos-or-calyxos/#additional-hardening