Brush libraries (Window > Brush Libraries > [library]) are collections of preset brushes that come with Illustrator. You can open multiple brush libraries to browse through their contents and select brushes. You can also open brush libraries using the Brushes panel menu.
To automatically open a brush
library when you start Illustrator, choose Persistent from the brush
library’s panel menu.
Copy brushes from a brush library to the Brushes panel
Import brushes into the Brushes panel from another file
Create new brush libraries