Comment by grapheneos

Software doesn't stop being open source because the releases are signed to be able to verify those. Any OS can be installed on the supported device. The verified boot key hash shown at boot will be different for an unofficial build and so will a query to the hardware asking it to provide it. Auditor and AttestationServer are open source. Those work fine with unofficial builds and forks of GrapheneOS after making builds of those with the correct key.