Single mouse click on a node or on a link displays it as selected. Multiple selections can be made in two ways:

select or toggle the selection by pressing Ctrl and clicking the mouse

draw a selection rectangle by pressing Shift and dragging the mouse

The Copy and Paste actions require one or more selected nodes of the same copy category (see below) and include only the links between them, if any. Paste is activated after the Copy selection and needs a location point from a mouse click. Any combination of the following works on selected nodes:

Copy and Paste buttons on the top toolbar

Copy and Paste menu items from the top hamburger menu

Ctrl+C and Ctrl+V from the keyboard

The copy selections are kept in the clipboard until Paste or cleared by pressing Escape.

The copy categories comprise nodes that can be copied together.
Flowcharts include five categories:

All central nodes

Start nodes

End nodes

Left/Top nodes

Right/Bottom nodes

Database ERD nodes are grouped in one category.
Copying between different diagrams is not accepted.

You can delete all of the selected graph elements using any of:

Remove Selections button on the top toolbar

Remove... menu item from the context menu of the graph element

Remove Selections menu item from the top hamburger menu

Delete key from the keyboard

Mouse click outside any graph element cancels all selections, except if pointing to a paste location.