Machining 101

Keep in mind when I ask this that I know less than nothing what so ever about machine work.

To adapt new and used barrels, change length etc. is it really as easy as dropping them in a lathe?
In other words, is this some thing that any machine shop could do if I tell them what I want or does it have to be a gunsmith for it to be understood?

I imagine that if I showed them a breech block and said “make this barrel fit this block” it would be straight forward but if I told them to “cut a 1/2” UNF thread in the muzzle and recrown it” would they understand what I wanted?