How to react with Models listed in Gazebo

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Hello, My System: Ubuntu 18.04 and ROS melodic I am new with gazebo_ros. I have opened gazebo using rosrun gazebo_ros gazebo and inserted turtlebot. the gazebo looks like this. Then I saved the world file and using the launch file below I can run it again: follower is the name of the package.

<?xml version="1.0" encoding="UTF-8"?>
<launch>
  <arg name="world" default="empty"/> 
  <arg name="paused" default="true"/>
  <arg name="use_sim_time" default="true"/>
  <arg name="gui" default="true"/>
  <arg name="headless" default="false"/>

  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="$(find follower)/src/worlds/follower.world"/>
    <arg name="paused" value="$(arg paused)"/>
    <arg name="use_sim_time" value="$(arg use_sim_time)"/>
    <arg name="gui" value="$(arg gui)"/>
    <arg name="headless" value="$(arg headless)"/>
    <arg name="debug" value="true"/>
    <arg name="verbose" value="true"/>
  </include>
</launch>

Now I am wondering what more do I need to use these models. Because these models do not offer any new topic to use them like for moving the robot or any sensor values as rostopics. I understand that this question may be repeated but to the best of my knowledge I could not find the answer for it.

For these models, that they were already inside the gazebo, what more files, commands, etc I need to move them and read their sensor values as rostopic?

In the world file there is no plugin. but it has the robots. here is the world file https://gist.github.com/yosoufe/a4bb6...

thanks in advance