I can't boot CC 2020 at the moment. It's not impossible that Adobe changed things in "bleeding edge" versions...but..
... in CC2017 (or earlier)
To apply the Scribble Effect
- Target the *fill if you wish the Scribble to be applied the fill.
Effect > Stylize > Scribble from the menu and the Scribble Options dialog will pop up, and apply the effect the to selected object.
- Within the dialog window you can make any adjustments you want to the scribble itself.
- Click the OK button when you are happy.
To alter the Scribble effect after it's applied...
- Look at the Appearance Panel (
Window > Appearance)
- Expand the fill (or stroke) and click the
Scribble item seen in the panel.
- This will open the Scribble Options Dialog window again where you can make further changes.
To alter the style of the strokes
within the Scribble Effect...
There's no direct way, in CC2017 at least, to directly alter the stroke appearance within the effect itself. The Scribble effect will only create simple, basic, strokes. There's no ability or method to apply something such as a brush stroke to the paths created by the effect while at the same time, keeping the effect "live" and editable.
However, you can expand the appearance to apply a brush stroke...
- Set the effect
Object > Expand Appearance from the menu. This will "bake in" the scribble and it'll no longer be adjustable.
- Now you can click a brush and it will be applied to the scribble paths...
If the effect has been expanded...
Lastly, if the effect appearance has been expanded via the menu command
Object > Expand Appearance, and all the lines are highlighted when you select the object... a la...
... Then there is no way to easily alter the effect any further. It is no longer a "live" effect in that case. The easiest solution for this is to recreate the original object/shape and then reapply the effect to the new object, deleting the expanded artwork.
These animations are using CC2017. There may be some visual differences in more recent versions of Illustrator. However, all the functionality should still be present in the options dialog window.