The default order in Tettra is by most recently updated, but you may choose to order by priority, or suggested reading order, etc.
All pages within a subcategory can be ordered in any way you choose from the subcategory view. To reorder a page, simply click on the drag handle (9 dots icon) on the left of the page title, and drag into the order that you would like.
Here is a demo gif:
This order will be reflected in all views of the subcategory, including the page side-bar. Once you set a custom order for the subcategory, new pages will automatically be appended to the bottom of the list.