These targets don't receive full monthly security updates, don't provide all of the baseline security features and are intended for development usage. These are extended versions of the generic targets with extra components for the SDK. The best development devices are the Pixel 6 and 7 series. It's not possible to work on everything via past generation devices. Newer generation devices have stronger hardware / firmware security and hardware-based OS security features and are better development devices for that reason. A fully signed user build for these devices is a proper GrapheneOS release. These are all fully supported production-ready targets supporting all the baseline security features and receiving full monthly security updates covering all firmware, kernel drivers, driver libraries / services and other device-specific code.
This is a guide on building, modifying and contributing to GrapheneOS as a developer.