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).
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.
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.