fixed side scrollbar for case study

This is my first post. I am new to code and am trying to code my case studies for my portfolio. I want to include a “scroll bar” to work as a sort of timeline on the left side that is “fixed” so as you scroll down the page it fills up. This website has a something close to what I am looking to accomplish. they have a small red “scroll bar” on the left side.

https://edesigninteractive.com/