Small correction, GrapheneOS are not making them. They are partnering with an existing large OEM to ensure one or a number of future flagship devices meets their security, privacy and support requirements.
It has been officially stated that GrapheneOS is partnered with a major Android OEM working on making devices meeting all of those requirements along with providing official GrapheneOS support. The devices are planned for 2027 but is being announced by the OEM in March 2026 so people will know which OEM it is soon.