It's really not that hard - the basic, but not only, difference between manufacturing and gunsmithing:
Gunsmithing: you are performing services on a customer's firearm, and charging only for your parts and labor.
Manufacturing: you are performing modifications to firearms you own to add value, and selling those firearms to customers after performing the work.
As long as you are doing work only on customer's firearms, it's hard to cross over into manufacturing.
If you also sell firearms, then the minute you start modifying those firearms before selling them, you're looking at crossing over into manufacturing activities.
There are some other areas, but the simple rule above covers at least 95% of anything you'll run into.
Example: You, as Joe FFL, buy stripped receivers and lower parts kits. If you assemble those kits and sell complete lowers, you've just crossed into manufacturing. If you sell the receiver and lpk to a customer, and you assemble them for them, you've just performed gunsmithing.
If you want to sell both stripped and complete lowers, and don't have a manufacturing license, buy the stripped as stripped and sell them that way, and buy the complete as complete, and sell them that way. If you choose to throw in the assembly for free, after the sale, then do the work after the sale, and not beforehand.