Select the nodes you wish to copy. You can do this by shift-clicking individual nodes or shift-clicking-and-dragging to select multiple nodes simultaneously.
Enter the keyboard shortcut your system uses for copy (Ctrl+C on Windows, Cmd+C on Mac).
You should see a message indicating the content has been successfully copied.
If you want to copy nodes from one flow to another, open the second flow.
Enter the keyboard shortcut your system uses for paste (Ctrl+V on Windows, Cmd+V on Mac).
The copied nodes will be added to the canvas.
Connect the pasted nodes to the existing nodes as desired.
Some nodes cannot be copied and pasted this way - for example, you cannot paste in a second start node because each flow can only have one. If the nodes you copied included one of these nodes, it will be omitted from the ones pasted in.