It is getting very confusing so let me attempt to clarify a few things.
It is prefectly legal to buy all the components to "build" an AR. The lower receiver must be transferred through an FFL as it is the "gun" and has a serial number.
If one wanted to build an AR from the ground up, order on-line and have transferred to your FFL of choice a lower receiver.
Then order or otherwise purchase everthing else you want to complete the rifle.
THAT IS NOT MANUFACTURING.
Put the parts together and you have rifle. No special engraving or forms are needed.
You can "put together" something of equal quality in your garage for several hundred dollars or less, legally using the exact same components. Except that "ferritic nitrocarburized" BS gas tube, heck, you could probably even find one of those if you really wanted.
And as there are exceptions to the rule, as I'm sure will be pointed out, very, very few people build (MANUFACTURE) guns from a solid block of aluminum, steel or what have you. Then ATF rules might start applying. If one is doing such I imagine they already know the rules, or damn well better.