Libraries use the selector of any step that calls them. If a calling step does not have a selector, the library uses the selector of the step's project. Libraries are typically called by other projects as an Inline for a step or as a Pass chain or Fail chain for a step.
You can change a library into a project by editing the project and choosing a selector for it. When you save a library with a selector, it becomes a project, disappearing from the Libraries list.
Aside from the lack of a selector, libraries are treated just like any other project.
To access the Libraries panel, in the left menu, click Libraries.