Work on the Layout that shows only the part you want to copy.
Use the System Track to select what to copy (you highlight the bars, then need to click the little box at the end of the track to actually make the selection)…
Then copy (ctrl-C)/paste(ctrl-V) wherever you want.
Dorico only copies things that are selected. But many items may be hidden, so not selected. The system track overcomes this. You can also select some hidden items by showing their singposts (View>Signposts). If they are visible, they can also be selected.