The Soviets never landed any troops on any major Japanese island. By contrast they had invaded Manchuria and there was a very real possibility that they could have taken all of Korea in August 1945; with some troops already entering the country.

At that point the United States asked that the Soviets not occupy the whole country; hastily proposing the 38th parallel as the partition point. The Soviets agreed to this and in fact honored the agreement; at least until Cold War tensions increased, the South Korean government proved somewhat unpopular and repressive, finally leading to the Korean War.

The soviets actually did ask for an occupation zone in japan but the US wanted to keep japan for itself. There was a plan to invade hokkaido but the US specifically warned them off (i.e. japan is ours). That goal is also the reason the US whitewashed the japanese war crimes trials, they wanted to maintain the japanese establishment to help them maintain the peace and that's the reason why the japanese are so intractable about accepting any kind of responsibility about events prior to '45.

As for korea, the americans were surprised by the speed of the soviet advance in manchuria and didn't have any troops available within reach before a soviet advance would've covered the whole of korea. Two minor diplomats sent the soviets a proposal about the 38th parallel in the hope that the soviets would agree.

It kind of was. The US occupied the 4 main islands of Japan--Kyushu, Shikoku, Hokkaido, and Honshu. But the Russians snatched the Sakhalin Islands from Japan, and they kept them. They still possess them. That was their Japanese occupation.