Select from page attachments option allows you to select any existing in page attachments diagram with the .drawio extension, insert it into the page and update it in the editor.
After you click the Select from page attachments Attachments button, the list of page’s attachments with the .drawio extension will be shown.
Selecting the diagram will open it in the editor.
When you have finished working with the diagram, click the Save & Exit button to save the changes and exit editor or Exit to discard the changes made (if any) and leave.
After that, you'll see the preview of the selected diagram.