The body of water that is between the main island or mass of Asia and Japan is the Korea Strait. However, other bodies of water may also be crossed. These include Sea of Japan (East Sea) or East China Sea really depending on the portion of mainland Asia someone is coming from to reach Japan.
Culture is the answer for your space.
When two tectonic plates push towards each other, one continental, and another oceanic, the oceanic plate is subducting under the continental plate. This is due to the fact that the oceanic plates's crust is weaker and has lesser density, while the continental is stronger and has bigger density, so when pushing against each other, the weaker, less dense, oceanic plate can not oppose the stronger, more dense, continental plate and slowly subducts under it.
The move because of the intense heat in the Earths core, that causes molten rock in the layer, called the mantle, to move. There are 4 different plates; collision, conservative, constructive and destructive. Hope this helped <3
