What is the Great Wall mostly built from away from Beijing?

Rammed earth. The photogenic stretch of stone and brick with watchtowers near Beijing is Ming work from the 1500s and is a small fraction of the whole; across the dry west the wall is packed earth, sometimes reinforced with reed and tamarisk layers, and long sections have simply eroded back into the landscape.

It is also not one wall. Different dynasties built along different lines over roughly two thousand years, and a 2012 government survey using archaeology and satellite imagery put the total length of all fragments at over 21,000 kilometres - a figure much larger than the older estimates because it counts everything rather than the Ming stretch.

Preservation is a real and losing problem. Something like a third has vanished, taken by weather, farming, road building and villagers using it as a quarry for house bricks, and remote sections have no protection worth the name. There is a small irony in the most famous defensive structure ever built being dismantled quietly by people who needed the materials.