From version nb184 (July 30,2020), we can copy components and screens in the Designer.
http://ai2-test.appinventor.mit.edu/reference/other/editing-functions.html
- Select, Copy and Paste one o more components.
- Select multiple components and change a Property.
- Select and Copy one o more components and Paste it as text in a text editor, message, ...
- Select Screen Copy and Paste it as text in a text editor, message, ...
- Select and Copy Screen1, create Screen2, paste Screen1 in Screen2.
- Copy multiple files using drag and drop.