How to start multinode in ignition gazebo fortress without roslaunch

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hi， I use roslaunch in classic gazebo to launch my world and model and launch files such as controllers. But in ignition gazebo, I want to leave the ros system and use the ign launch that comes with ignition gazebo to launch my files. However, I found that the .ign file is not the same as the .launch file format, and I have not found any relevant information to rewrite my launch file, the .ign file I know only contains <executable><command> and <plugin>, what should I do?