How to put a model on a world created and export by blender?

edit

Check the SDF of models if they have set

<collision>

tag. You can set the collision geometry to the same as model, or simpler. If all your models have correctly set collision geometry, mass and other physics attributes, they should behave like you would expect and not fall through ground.

Also, you want to set ground's static parameter to 1:

 <model name='ground_plane'>
  <static>1</static>
  ...
 </model>