If you right-click on your page in a current OneNote section, you can select Subpages | Make Subpage to turn that page into a subpage of the page just above it.
When a page has subpages, it will have an arrow next to it so that you can use it to collapse and hide. Oh yes, and don’t I love having that bit of organization!
Subpages can have subpages, but only top-level pages can collapse. You can turn a subpage into a normal page by right-clicking it and selecting Subpages | Promote Subpage.