As long as the lower receiver mates to the upper and positions the internals properly, the process used to make it is a non-issue. In theory, you can exchange forged and billet receivers of multiple makes without issue...in practice, of course, different specifications, tolerance stacking or different tolerances all together can create mischief. There's a tremendous amount of standardization for AR-15s and it's pretty amazing...
Lets do this conversation with bolt guns for fun:
"So, what I want to do is take an McManners stock inletted for a Remington 700 action, throw a Sako action in it, and then mate that up with a Savage barrel, and a AICS magazine with Winchester M70 bottom metal and a CZ 550 trigger."
Because that would work, right? Yet many of us do the equivalent of that exact thing with our AR-15s and it works just fine because almost everyone builds key specifications to the same dimensions most of the time.