I am not a Photoshop Pro. Just know the basics. And never have tried digital drawing.
For this I had created a sketch first on the page. I can't do the method which Roman followed. This is a perspective drawing. Having a basic knowledge of perspective helps.
I have used Kyle brushes, which are free if you have adobe subscription. I took time to get a hand on brushes. I didn't create this in one go. Took days to design it and, also, took some illustration ideas from Roman's actual sketch.
Following videos will give you a better understanding on how it works.