I created this design several years ago and I can't find the .ai file!!! I just need to change the background so I'll have to re-do it, but I can't remember how I created these lines. They were originally straight simple lines (line tool) and I don't know how I turned them into these...
You can drag a shape to your brushes panel and make it new pattern brush. After that select a line. Then click the new brush in your brush collection.
Make one horizontal line and apply the new brush. You will make the pattern overlapping itself by expanding the appearance and adding a stroke.
Rotate & copy the new-shape lines as will.
a random closed shape - only a fill, no stroke! Dragged the shape to the brush collection and marked to be a pattern brush
the same line after applying the new brush
The same line after going to Object > Expand appearance and after that selecting a stroke for the shapes to make overlaps.