Roll mark ?
 

I'm looking to get my first black rifle. I'm very familiar with traditional long rifles, but at a lost with ARs. I was looking at quite a few in the local shop. Different brands, barrel lenghts, calibers.... and I asked in his opinion who makes the better or best AR (noob question, I know). His reply was to direct me to a keyhole looking roll mark on the upper right of the receiver. He proceeded to hand me at least 4 different makes of rifles. Some were the least expensive, some were his most expensive, all had the same roll mark. He then stated the receivers were all made by the same shop. He basically said pick one.
My question, is this roll mark the mark of the machine shop making the receivers or are all ARs marked as such?

The mark is the forge mark, the forge sells to many companies to machine then those companies sell to many more assembly companies. Most companies even big ones like S&W buy many of their AR parts, S&W buys some if not all of their barrels from ER SHAW, their uppers seem to be YHM with the keyhole forge mark, seen the same forge mark on Daniel Defense uppers.

That explains it. Is the receiver milled by the same company that forges them?

No, some assembly companies mill their own but most buy them from DOD contractors.