While the checkout page is a system generated page - there are options to add content to various areas of the checkout page.
Above: The content areas available to add custom text. If an area contains content it will be shown in blue italics.
If you would like to add form fields to the area for additional questions you can drag-drop snippets into the relevant content area.
Either select the FORM FIELD dropdown from the sidebar, or click the green ADD SNIPPET button in the main editor area, and then click the FORM FIELD button on the left.
You can add different types of form field to the page, the most used being the checkbox option as shown below:
The screen above shows dragging a single checkbox field into the page.
With form fields you should edit the field name to something understandable via the blue EDIT INPUT button that appears when you hover over the field in the editor.
The answer will then appear as additional information on the order.