env.yaml specifies the configuration settings for four essential components of the simulation scene: the environment, the robot, the sensors (on the robot or in the scene), and the assets (obstacles, interaction objects, etc.) The below provided sample scene configuration file initializes a Unitree Go2 with a set of specified sensors in a standard warehouse environment. 
scene:
  - scene_name:
      type: Asset
      name: Warehouse
      config:
        prim_path: "/World/ground"

  - robot_name:
      type: Robot
      name: Unitree_Go2
      config:
        init_state:
          pos: [0.0, 0.0, 0.0]
          rot: [0.0, 0.0, 0.0, 1.0]
  
  - sensor_name_1:
      type: Sensor
      name: HeightSensor
      config:
        prim_path: "/World/envs/env_.*/robot/base"

  - sensor_name_2:
      type: Sensor
      name: CameraRGB
      config:
        prim_path: "/World/envs/env_.*/robot/base/imu/cam_rgb"
        height: 256
        width: 256
You can also set scene specific variables like number of parallel environments, decimation, time step for simulation, episode length (in seconds), etc. using env.yaml by adding the following to it. 
num_envs: 1
decimation: 4
episode_length_s: 5
env_spacing: 15

sim: 
  dt: 0.005
Finally, the env.yaml also configures the ROS2 communcation layer. Please have a look at this page to get more detailed information on how to publish simulation data or subscribe to commands.