On further considering, scanning equipment of some kind was likely employed by the Excavator group(s) who likely spotted the easiest entryway to this construction. This entry way may not have been the original intended entrance or one of many entrances to the building(s) complex. I propose a pocket of air was identified, allowing them to insert numerous boring tools and they were then carefully able to expose these spaces. Understanding this, we can infer they were able to excavate more sections but did not excavate it completely - rather to get the key artifacts or items of interest. The right hand side "tunnel" that appears to be partially excavated may have been deliberately filled in after the site was reclaimed, repurposed and re-attributed to different people.

To identify a building that is encased inside a hill or under the ground requires scanning equipment, so this was likely a former technological civilization that came AFTER the true builders of these columned works died out or were killed.

Remember, Petra site is a massive complex that goes further underground and was built in the OPEN AIR, above ground. It was a mound, no one would know there was a building there unless they had knowledge of it somehow. Even if they knew of a place with artifacts, they would need tools to accurately measure and excise the materials safely without destruction of chambers that still had pockets of air or were easiest to excavate.