To build a library. Save the blend file in the asset library folder. In the blend file you will need to mark what you want as an asset. This can be done in outliner thru right click menu. You can also do it thru the name. For example for a material goto the material tab of properties editor or the shading editor. Were the material name is you can right click and mark as asset. You can use name field to mark most things as assets. Such as animations, objects, node groups, etc
In outliner you can select multiple objects, collection or materials and mark them as assets . Note: you can use the filter (funnel icon) to show only materials. This makes it easier to mark a large group of assets.
