Enable the developer options menu by going to Settings ➔ About phone/tablet and repeatedly pressing the build number menu entry until developer mode is enabled.
Next, go to Settings ➔ System ➔ Developer options and toggle on the 'OEM unlocking' setting. On device model variants (SKUs) which support being sold as locked devices by carriers, enabling 'OEM unlocking' requires internet access so that the stock OS can check if the device was sold as locked by a carrier. Connect the device to the computer and install Grapheneos via browser.
You need to boot your device into the bootloader interface. To do this, you need to hold the volume down button while the device boots. After install OEM was lock again.
Arch + Hyprland on my Notebook, Endeavor OS + Gnome PC (11years old PC), 2x Khadas VIM3L + Kodi (Coreelec), home server Odroid + Armbian.