/ic/ - Artwork/Critique

Alright I need some advice. I'm trying to follow FZD's advice in his 101 vid which looks pretty good, but there are some things I don't get which I tried to describe in the pic attached.

Left is ref (screen grab, sorry for the quality)
Mid is my attempt at drawing it
Right is his process, with the sketch, the line weight and then the contour lines + details added.

People have told me my drawing are too clean, and I get it but I don't understand really how to make them livelier and more chaotic. I've circled in red the areas which I don't understand.
In the sketch, where are the scribbles coming from?
In the line weight, I get that the darker areas are darker because they are in shadow in the ref and they delimitate the shape of the branch, but why does he add those seemingly random lines? How does he know where to put those lines?
The last one I get it's mostly adding more line weight and contour lines in the way he thinks is necessary to describe the form.

Any help on how to sketch in a similar way?