Which process is more advantageous?
both have their own merits and demerits,
milled receivers are generally more sturdy and makes an AK balance sligthly more well due to proper weight distribution so muzzle climb on it is somewhat easier to control,
but since every receiver is made by cutting off material off a solid block (i.e. 'milling') it requires more material, a proper manufacturing fab unit with all these machines and trained labour for that,
hence it's costlier,
also milled receiver are hard to modify too once they are done, so god forbid if you need to drill out holes to affix new things or something like that...
BUT if done properly you could expect some of the best kwality build a weapon could get,
^AR is a milled receiver design
now stamped receiver design hhmm it's basically process of using milled parts only where required i.e. front and rear parts where other things are affixed on the rifle rest ;body; is made from specifically folding metal sheets in shapes using pressing machines (i.e. 'stamping machines'),
this is a cost-effective solution of making machine bodies as material required is less, manufacturing could also be made faster and trained labour required is also lesser, and modifying a stamped receiver is easier, a normal repair shop level tools are enough,
disadvantage though is as expected, weight distribution here is not so good and sometimes, low QA of the manufacturing could potentially result in average kwality builds with mismatch in shape etc,
^AK of the current era are mostly stamped sheet receiver design
NOW some historical thing,
very first AK-47 was a stamped sheet metal receiver design but Soviets had issues in mass manufacturing it, rejection ratio of the receiver made using ww2 era stamping procedures they followed was quite high for this type of automatic rifle,
they had to go back to milled receiver designs in 1950s to at least keep continuing making those rifles while they initially brought german POWs like Surmgewehr fame Hugo Schmeisser and some others to improvise their stamped metal manufacturing processes etc, Schmeisser later went back and died in 1953, it took them about 5 more years to finally improvise-standardise stamped sheet receiver manufacturing of the AKs and hence came AKM
^This is also related to general perception that AKs are easy to manufacture blah blah...it took Russians themselves about a decade to properly make a stamped receiver AK that often required a factory to first build different parts and then assembling it all together,
while they were able to make milled receiver AKs without much issues using whatever they were having back then, just that it costed more etc...
...
so stamped receiver AK mass manufacturing only becomes cheaper IF one is ready to invest in a manufacturing facilities of the parts for that, OR able to source ready made parts for assembling it all together, and then there also remains an issue of maintaining quality of the builds...