My response is purely federal law based and has nothing to do with any state laws or definitions. It also has nothing to do with the current 80% receivers, receiver kits, etc. This is all very basic and clear and not muddled by current events or political crap.
Federally we have a taxonomy with Firearm covering everything. Firearms can be different types with the obvious ones being rifle and pistol. If you decompose a rifle you usually have an object that is still a firearm called a receiver. A pistol maybe be made from a frame or receiver. The key concept here is that if you have a functioning rifle or pistol (or anything else) and decompose it, there is always some basic element that is the firearm until you decompose too far.
So if you are holding a receiver which is a firearm and marked, serialized, etc, you have a federally regulated item. Let’s say for this discussion it is Anderson AM15 stripper ar15 lower. If you take this and complete the lower by putting in a LPK and buffer and stock you still have a receiver and therefore what you have done is “gunsmithing”. The object has not changed state as it was a receiver and still is a receiver.
Now you attach a complete upper. The object you are holding is now a rifle. This is a change in taxonomy from a receiver to a rifle. This constitutes manufacturing. With the exception of manufacturing that is decomposing, you must mark the item when manufacturing. This is a FFL requirement we are talking about. Any manufacture must mark what they make. So you would keep all the markings you can but the FFL must add business name and business city and state. They must also record this manufacturing in their bound book.
If you turn a rifle into a receiver, this too is manufacturing. It must be recorded in your bound book. Because you are decomposing an existing object, there are no marking requirements as long as the resulting object is properly marked.
All manufacturing requires a proper FFL, an 07 in this case.
As a private party you have no requirements to mark or record what you make. So when you put an upper on a lower while it constitutes manufacturing, there are no federal laws that require marking, licenses, etc UNLESS you are doing it for profit. Private manufacturing for personal use is fine. Manufacturing for profit or for someone else even if not for profit requires a license.
You can help your friend build an AR. You can turn the stripped lower into a complete lower. You can build a complete upper. You cannot put an upper on a lower for someone else. Sure, this is very unlikely to get caught or cared about, but it constitutes manufacturing and you cannot do this for someone else without a license.