How To create layers from portions of another.

You could select the layer that you'd like the transparency in, then use the rectangle tool to mask the rectangle. You would have to go in to the timeline where it says mask one and check inverted.

Place the video with the skateboarder underneath this layer in the timeline.

If you like you could also select the layer that you masked and type the keyboard shortcut "f" and adjust the blue numbers by feather to soften the edges of the mask.

Sorry, I don't understand your intent.  Is English your native language?  If not, you can type in your native language and we will use the translator feature.

I'm not sure if I understand your question but if you have a shot of an empty scene and the same shot of a scene with a person walking you can put the empty scene above the scene with the person in the timeline. You can select the empty scene and double click the rectangle tool to create a mask outline. You could then double click on the colored outline and this will allow you collapse (drag) the left or right side of the top layer to where you want the person to appear from.