Read through to learn more about:
Adding an asset to the library
Enabling the asset library
In order to add an asset to the library, click on the dot menu on any asset and select the option 'Add to Library'.
To access the library, click the view option at the top and then select 'Show Library'. This will replace the asset panel with the library, which will show all the assets you have added to it.
You can upload and insert assets into a creative from the library exactly as you would when using the standard asset panel.