To move a Layer Mask from one layer to another, click-and-drag the mask to another layer. To copy a layer mask to another layer, press-and-hold Alt (Mac: Option), then click-and-drag it to any other layer.
One way to duplicate layers is to hold Alt (Mac: Option) and click and drag on the layer thumbnail in the layer's panel. You can release the mouse button anywhere on the layer's panel to place the duplicate in between whatever layers you want.
To activate the mask on the currently selected layer press Ctrl \ (Mac: Command \). This is the same as clicking on the layer mask thumbnail. To activate the layer press Ctrl 2 (Mac: Command This is the same as clicking on the layer thumbnail.