Six years ago I retired from my job as a civil servant for the federal government. My job? Contract specialist buying stuff!
Procurement is done in accordance with the Federal Acquisition Regulations (FAR). It is further interpreted for the Defense Department by the DFARS (Defense Federal Acquisition Regulation Supplement). Each branch has its own supplement and further documents providing guidance and interpretation for buying stuff.
The underlying philosophy of the federal acquisition process is that the government buys from small businesses almost exclusively.
We buyers went through life blithely assuming we were supporting American small businesses and at the level I was working it was true. Most of our contracts went to literal small businesses. At the higher levels though the definition of "small" got a little fuzzier. My last job was to help manage the Base Operating Support Contract her at the Navy Base on Guam. This was a $350M contract run by a (wait for it) small business.