Reference frames/ Coordinates frames in gazebo

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Hello,

I want to know the various available coordinate frames or reference frames in gazebo and how can i transform the position of the model from one frame of reference to another?

background on the question: i have spawned the robot in gazebo, along with that i have also imported some models of objects. i want to perform the pick and place operation. Below is the image of the scene in gazebo. For doing pick and place, i want to know following things as well,

1. When i get pose of a model from gazebo from service call to get_model_state, the pose of the object is with respect to which reference frame??

2. Similarly, the pose of the robot is with respect to which frame of reference??

3. For pick and place, should i convert pose of the model in any other reference frame?

4. How to convert pose of the object from one reference frame to another?

Any kind of help is appreciated.

Thanks, Vishal