Help! about /setpoint_raw/attitude

I am using a project that implemented an environment to simulate drones and control them. I saw that it use setpoint_raw/attitude as the final command. but in this command, it set frame_id = ‘base_link’. but rqt I saw that the uav_fu does not connected to base_link. So I am curious how the command setpoint_raw/attitude work with base_link?
Could anybody help with this? I am in an emergency project.
Appreciate any help if it does.