They are stored in AI format files.
They are accessible only via the Illustrator brushes panel. If you want to create a permanent scatter (from an artistic) then you have to open the source file. If you make any modifications to the brush inside another document, the changes are only for that document (though you can always use the save command to save the document and brushes for future use in other artworks. The artistic strokes are the ones displayed in the preset panel as a long line, the scatter ones are the small icons at the top of the panel.
Once you have opened the Illustrator brush file and have the artworks available in the brush panel, you can select an artistic stroke and drag the brush from the panel onto the artboard. If you select a fill from the color panel, the entire path will be filled with that fill as the 'entire group' and that will result in a pretty useless stroke. You must either double click the brush group and enter the isolation mode / editing mode for the path and you can then select the individual paths and change the color and positions as well as add or remove paths. You can use the object menu command and ungroup the group and select the bounding box and then delete. Now you can modify the artwork in numerous ways such as adding additional paths or perhaps tweaking the anchor points of the artwork. Of course, you don't have to edit the stroke at all but it is always great to have a few variant sets for future use. You can also copy the modified artwork into the CC libraries so the item is saved to the creative cloud.
On finishing any modifications, you tcan hen drag the art back into the preset panel and select a new type for the stroke such as "scatter" or "pattern" etc. If you are editing the original source document, please remember to save the file otherwise all the changes will be lost.