How do I add my own custom airframe?

If you add your own custom airframe to the build and build the .px4 file and flash it (e.g. through make upload px4fmu-v5_default) it will be fully configured, but the meta information for it has not been provided to QGroundControl. To store the meta information in QGC you need to flash .px4 file through it - go to Firmware Upgrade, select the autopilot and PX4 and select “Custom File” instead of the latest PX4 version.

As mentioned in the developer guide Adding a New Frame > Add New Airframe to QGroundControl