I have one package with a platform and a few images displayed here and there and I want to place a robotic arm in the centre so I found many many open-source robotic arms to use. but they all load into different simulators how to display them in one single gazebo simulator and control the robotic arm.
welcome to the community!
I think what your need is create a custom Gazebo world that contains all the objects that your want in your simulation. You don’t need to create those objects yourself, you can take them from other open-source projects and include them into your Gazebo world.
How to create a Gazebo world is quite a broad subject to discuss in detail here but there is one course called Mastering Gazebo Simulation and Unit 4 covers that subject.
If you want to place your robot arm on a stand you are better off if you model that using URDF files. For that subject there is a course called URDF for Robot Modelling.
I hope I was able to guide you a bit on what it is what you need to learn.