Notes are organized in notebooks within libraries. You can create as many notebooks as you like within a library. This is helpful to organize notes within libraries by project, by sub-team, or organization. A "General" notebook is created by default for you. Right-click your library in the sidebar and select "Add Notebook" to create a new notebook within your library.
You can't create sub-notebooks in Notejoy. This is specifically by design to simplify organizing and finding notes in Notejoy. Since all team libraries & notebooks can be easily browsed in the sidebar, it makes it far quicker to navigate and browse content compared to traditional folder & sub-folder structures. It also makes it much faster to add notes since you don't have to drill deep into a folder structure just to create a new note in the right location.
If you are finding that you need more organization hierarchy, consider up-leveling your structure. For example, if you currently have one team library for your entire company, consider creating one team library for each department so that you can have multiple notebooks per department. Or alternatively, add hyphens to your notebook names to create sub-divisions: "ex: Product - Q1"