@Ana Ximenes - To uncouple/detach lines from shapes/cards/sticky notes/etc., select the line and then  click-and-drag the white dots to remove/attach it.

And to create new lines, click the object, and then click-and-drag the little blue dots that appear:

To make more shapes, you can copy-and-paste existing ones using Ctrl/Cmd + C/Ctrl/Cmd + V.

You can also create new objects by duplicating existing ones using the Alt + drag or Ctrl/Cmd + D shortcuts. See all shortcuts in the Shortcuts and Hotkeys Help Center article.

Thank you!