Beginner To ROS and PX4

I want to develop a precision landing using aruco marker using drone in gazebo classic , im currently using ROS2 Foxy and Gazebo Classic 11 , how can i do that i one weeks.
How to i have access to the downward facing camera in PX4 using ROS to do the OpenCV .

And how to i use my custom created world in gazebo with PX4

Thanks in advance