China and Russia’s advanced and partially automated and augmented defense systems would scramble, hack into, shut down, disable, redirect, or outright destroy or prevent nuclear missile launches. Drones would hack into and shut down facilities or weapons themselves. Infrastructure could be shielded and damage minimized in various ways, and supply chains are something that Global South understands intuitively more than the Global North.

I'm sorry but this is not based on any evidence. No nation including the US has this ability. Among other problems for China and Russia, the US systems are pretty old and hardened, they don't have a lot of attack surface, if anyone is going to get their nuclear system hacked and shut down it would be a power with a more modern system (I'm not sure if that describes either China or Russia). US has the largest number of nuclear missile carrying submarines constantly stationed around the world, roving undetected under the waters, waiting for the order to launch 20+ ICBM's each, each carrying 20+ MIRVs, each carrying a warhead. They can park off Russia or China and have their missiles launched and detonations within 10 minutes. The systems for signaling those are very simple, US mainland land-launched weapons also have dedicated hard-line communication lines, they don't use civilian internet or phone infrastructure.

As to intercepting, no nation including the US has more than a few dozen hypersonic kinetic kill sled anti-ICBM weapons to my knowledge. The US has by contrast over 4000 warheads. Even if they launched only a 1/8th of them on the basis that one cannot reliably count on shooting down a warhead with less than 2 interceptors, that's 1000 interceptors required.

You can't plan around physics other than deterrence. Maybe in 40 years with practical laser or particle weapons systems existing in large numbers you could make it impractical but the science simply isn't there yet. If the science were there already the US would be pushing full steam ahead with their own programs so they could strike first and shield themselves. Fact is though that with evasive maneuvers the US's own tests for success rates with their interceptors are rigged to look more rosy than they are. I'd bet China has better results and Russia as well but it's probably not enough because you have to have the numbers.

As to hacking and scrambling, if Russia could dominate in this way they wouldn't be having the problems they have in Ukraine, they'd have done more damage to their infrastructure, they'd have crippled their defense systems, but CIA/NSA hackers are there with them helping them defend against Russian attacks very successfully.

Let's not forget the US has a starting edge here too. They infiltrated deep into China's systems with their Cisco hardware implants and who knows if the Chinese ever rooted all of it out. I'm sure they're not able to actively spy on them as they were because that would be observable and detectable but that doesn't mean they don't have buried in there, inactive, waiting for a special command, some sort of malware that will shut down and destroy their command and control when the moment of total war comes. China is actively fighting a variety of US attacks on it, the US has been paying Chinese civilians with free hobbyist equipment that serves as CIA/NSA radio-equipped attack platforms which China has been trying to round up among many, many other plots no doubt. By contrast the US freaks out if it sees a weather balloon from China so I doubt the Chinese have the same ability. The US is banning Chinese technology on the grounds it could be used by China for electronic warfare and hacking, I doubt the Chinese intended to do this but I think it's projection and shows US plans and projects already well underway.

On shutting down these weapons. They are specifically shielded against EMPs because one of the earliest concepts of nuclear defense was using nuclear detonations in the atmosphere in the path of incoming warheads to attempt to destroy, misdirect, and otherwise neutralize incoming enemy missiles. (See for example NIKE missile program)

As to scrambling, they don't rely entirely on GPS, their paths are calculated using mathematics that don't require active pinging of positioning systems in orbit, after all these were first developed before GPS was even a dream, let alone a reality and had to be able to get to their destinations.

As to decapitating strikes on the US, they have a fleet of always in the air emergency command and control aircraft specifically for the purpose of ensuring that the orders can be carried out (in fact I recently saw an article where the air force is looking to replace their current fleet of these. It was originally called Operation Looking Glass). (Russia by contrast has a system of several missiles which are programmed with emergency launch codes which can be launched and will travel the length of Russia blasting those codes and ordering all warheads to launch called Deadhand. China to my knowledge has no such system and relies on moving their warheads around in secret and plenty of mobile launchers to simply make it harder to hit them all in a first strike sneak attack but which does little for command redundancy)

So I'm sorry but if the US starts a nuclear attack in earnest with any significant number of weapons, the only solace that China and Russia will get is that their own nuclear weapons will destroy American military bases and burn their cities to cinders in retaliation. I find it improbable that one would be intentionally started even by the US, the real risk is an inability to back down and backing Russia into a corner where it has to use nukes.

As to nuclear war's effects. I recommend the movie Threads from 1984 which went to great pains to be factual. There's a saying among nuclear war theorists and planners and it's that those who die in the atomic blasts would be the lucky ones. And that's because such widespread destruction would cripple industry, food, clothing, manufacture of energy, medicine, etc for decades. Tens of millions would die not from fall-out but from starvation, from deprivation, from cold, from heat, from previously treatable diseases and epidemics which would rage in the kindling of such destruction of cities.