To copy and paste content, for example copying handwriting from one page to another:
1. Select the Lasso tool. Tap it again and make sure to enable the relevant options. For example, if you're going to select handwriting, check that the "Handwriting" option is enabled.
2. Circle the content you want to copy/move.
3. Tap on the selection and tap Cut or Copy.
4. Long-press on the current page, or any page in another GoodNotes document, and tap Paste.
Note: it's not currently possible to drag a selection to the previous/next page.