If the USA gets directly involved in Korea, China and Russia will as well, and unlike the USA they have a direct land border.
The Russian military is too committed to Ukraine to make more than a token commitment to a war in Korea, and given the wretched showing they've been giving against the
Ukrainians, after US air power severed their rail connections across Siberia, they'd be more or less a non-factor.
If China went in on North Korea's side, that
would make a meaningful difference, but also almost certainly bring in Japan, Philipines, Vietnam, Australia, and quite possibly India.
They'd be able to inflict casualties, but even if they performed halfway between the current Russian standard and the standard of an actually competent military (unlikely, odds are they're at least as bad as Russia), they'd get crushed.
Even if they performed to the level of an actually competent military, their sharp technological disadvantage would almost certainly result in them losing. They do have
some fully modernized kit, but the vast majority of their military inventory is old,
inferior knock-offs of Cold War Soviet equipment.
China
might be able to win a defensive engagement against US & Allies, but that'd almost entirely rely on massive networks of defensive AA platforms. Fighting in Korea, where they would
not have that advantage, they'd get bloodied, and badly,
very quickly.