How to Copy and Paste selected content?

There are copy/paste btns on the selection tool's option panel(up and down arrow btns). And there are menu entries in Menu->Edit too. You can copy content from one sprite to another file.

If you want to copy the whole canvas, you do not need to select the entire canvas first, just use the menu entry's copy.

If you want to copy frames/cells, you should use the copy/paste of timeline. Double tap on cells, you can find the copy/paste of cells feature. And frame/cells support multi-select. That's means you can copy batch of frames/cells once, and supports copy through two sprite files too.