A line drawing is made using the pen tool in Photoshop to create a path that is exported to Illustrator for editing and sizing. It’s important to avoid tracing. Just use the assembled photos as a guide.
If the final illustration will be a wire sculpture, the drawing is made with as few continuous lines as possible. The drawing above was made with three continuous lines. In illustrator I might reduce that down to two lines and when I make the final wire art, I might be able to do it with one continuous piece of wire.
If the plan is to make a botanical illustration or stained glass piece, the drawing is made out of many shapes placed together like building blocks. Each shape will hold a different color when the illustration is complete. All the shapes together will be used to create a mask that is used to remove background.