China would punish both, since neither country could ever be trusted economically ever again, and China would like to continue trading with the rest of the world which would be objectively impossible. Nuclear first strikes are war crimes, and ones that pretty much mean instant global sanctions.
Now would be the time to test that, given the EU switching over to their own payment processor and China giving the other 2/3rds of the world a payment processor that's not Visa/Mastercard.
Everything is set up in such a way that both Russia and the US can be effectively removed from global society with the only danger being them nuking a nuclear power; which even the most insane sycophants wouldn't do, since that ends the world. If Israel/US nukes Iran, the US will never trade again with any country or entity, the USD collapses literally instantly as the US has no assets backing up its fiat currency BESIDES their economic and military stability, and a whole lot of US military members will mutiny if not actively revolt.
The same is true of Russia, but to a lesser extent since they've effectively been in isolationist manufacturing mode since 1991 so they're far less reliant on global trade, still a lot of Russian billionaires would revolt.
Every single key to power in both countries is against nuclear escalations, since that ends their power globally. Rubles and USD would be worthless, their assets would be worthless, their foreign assets would be seized, and that's if they survive popular riots/revolutions long enough to flee.
There is no threat to justify nuclear force, for either country. There is no possible way to get a plurality, much less a majority, of people to defend the actions. And again it would make all the people involved poor.