The Russia – North Korea border is at the Tumen River that opens into the Sea of Japan. China’s border is at the Friendship Bridge upstream.
The Friendship Bridge connects the Russian town of Khasan with the North Korean town of Tumangang.
To get to the open sea, Chinese goods cross into North Korea by road. Then overland to the port of Rajin further down the coast, a port that was built by the Chinese.
To reach the Sea of Japan by sea, Chinese ships have to leave from Shanghai or Dalian and travel up the strait between South Korea and Japan.
Just this summer Russia proposed that they and North Korea allow China to navigate the Tumen River to the sea.
If that was all that was proposed it wouldn’t make much difference to China. The river is too shallow to allow large ships to pass. And the Friendship bridge is too low.
But the proposal is to dredge the river to allow larger vessels to navigate it, and to demolish the Friendship Bridge. And presumably to rebuild it but higher to allow large ships to pass.
Large ships would include the Chinese Navy. If the project happens it would mean a strategic change in the region.