You can copy and paste any content from a Goodnotes page, to be used in another page or Notebook:
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 Handwriting is enabled.
2. Circle the content you want to copy/move.
3. Tap on the selection and choose Cut or Copy.
4. Long-press on the current page, or any page in another Goodnotes document, and tap Paste.
Note: it's not possible to drag a selection to the previous/next page.