Pastes whatever nodes you most recently copied. It is possible to paste nodes into a vector shape that have been copied from another vector shape. The nodes you paste will be selected when you first paste them and may be moved as a unit by left clicking on one of the nodes and dragging them to where you want them. In the figure directly above I selected only two nodes of the circle, but by doing so I copied the whole circle, so that's what I got when I pasted. They should all turn black and you should be able to move or copy them as a unit.
Select none releases all current node selections.
This command can be used to close shapes or for joining separate curved sections together into a closed shape. The type of join you get is going to depend on the types of nodes your joining. Here I got a smooth joining curve. Merge can be used on a selected node or nodes to essentially remove the control points. The nodes on either side of the merge node become connected together.