China can literally shoot dozens (let's not kid ourselves, hundreds) of drones, missiles and bombs into Japan and South Korea. Both have limited means to defend themselves against such an overwhelming threat and even less so the means to shoot back in any meaningful way, South Korea would prefer too keep their stuff in the backhand due to the DPRK and Japan just lacks serious offensive capabilities that are survivable against the numerical and technological advantage of the PLA.
As for Russia, they outproduce the entire rest of Europe with regards to shells, mines and drones.
View attachment 777151
(Lancet strikes according to Lost Armour)
Russia used nearly 3000 Lancets alone since December 2022 according to Lostarmour. With production being certainly higher. Russia is on a quest to launch 1000 Geraniums per day, with recent raids having consisted of several hundred drones already, which also double as decoys for the likes of Iskander-M, of which they produce like 70 a month.
View attachment 777161
(Geran launches according to Kyiv Independent)
This doesn't touch on stuff like MLRS munition or the amount of shells across all calibers produced. So given that a Sino-American War would either take place after the Russo-Ukrainian War is concluded (Alternatively it would at the very latest conclude then as US support for Ukraine would dry up completely with their attention being completely on the chinese), it means that Russia has all the facilities ready to supply the DPRK with such systems (referring mostly to drones and shells here). That isn't even me touching on the fact that China literally owns the FPV drone market, so they could unironically ship hundreds of thousands of these around, while producing millions themselves (obviously less useful in a naval war against the US though).
Also, why would I ask Iran? I'd rather ask Algeria, who got shiny new Su-35s and seemingly Su-34s as well delivered, allegedly having ordered the Su-57E too. If you actually place an order, it seems like you get what you bought. If you just buy Yak-130s, you get Yak-130s.
To conclude this, the overarching problem is that countries liked China, Russia and the DPRK have an incredibly robust industrial sector, with China being world leader. They can produce munitions and drones cheaply in large quantities. The US, Japan and South Korea/Australia are more so cozy service based economies, where goods are produced at higher prices and in lower quantities.
The US especially has forgotten to build cheap and plentiful, while having a habit of gold plating everything, I think it's something you can agree on. That used to be different, in the first half of 20th century the US was what China is now, industrially speaking.