You can turn any regular document into a Library in a couple of different ways.
Creating a Library in the Mac app
You can open any local or Workspace document and choose File > Add as Library… to turn it into a Library and add it to your Libraries.
Creating a Library in the web app
To turn a regular document into a Library in the web app, click on the three dots icon , select Settings… and enable the checkbox in the Use as a Library section.
This will mark your document as a Library for everyone in your Workspace.