If North Korea does, why not China?

Also worth mentioning that without the United States the present continental European militaries would struggle even against the battered ground forces of Russia. Can't really fight back with GDP of your service economy alone.

North Korea is being being paid by Russia to supply troops. Russia cannot afford Chinese troops. And even if they could afford them, China is throwing its weight around Asia and wants its military intact there.

North Korea is involved in it for the same reason countries send military observers to conflicts.

It hasn't fought a war in decades, and it needs to figure out whether or not any of its shit/doctrines/etc works. It doesn't actually give a rat's ass about Crimea or Ukraine or Russian claims.

It fully relies on friendly logistics to participate in the conflict.