I used a Pentel Brush Pen for this, a nifty little cartridge-fed small nylon brush, configured like a fountain pen. I own three of these and this one has the most non-pointy brush tip. It’s the one I use to fill in black areas on other drawings, so I couldn’t get the ultra-fine lines like the other pens. But, it still works well.
This drawing is totally from my imagination and actually turned out to be very “design-y”.