Library is an important feature inside the Editor, so it has a section of its own.
You can open the Library by clicking the icon in the Topbar.

The Library will show up

The Library contains 2 tabs: Sections and Saved.
Section tab
This tab has many pre-made sections that you can import directly into the page.
In the left, you can quickly go to section category for easy finding the sections.
Saved tab
This tab contains all the sections you have saved for later usage

You can rename and remove and saved section from their context menus.

You can save a new section by clicking the Save section shortcut of element inside the Viewport.

or right click on the element and select Save section

And that wraps up an overview of the Zeno Library.