Each burner is different. Each manufacturer bases the recommended clearances after testing to combustibles by the specific burner. In the case of the featured picture, the Key West burner (KWB30-N1) was used which has very low clearance requirements compared to most products in the marketplace. In all cases you should consult with your local supplier who should know any local codes.
Agree, you have to look at the installation and clearance details on each burner. Many can be installed under a pergola, they typically require the pergola to be a certain height away from the firepit.
Firegear Outdoors
Dekko Concrete