Sticky to scroll behaviour

Hello! I would like to know, how to achieve this sticky to scroll behaviour, like this website: Reddit Ads: Facts That Will Surprise You About the Platform | Agorapulse.

As you can see abothe fold, there is an image and blow that, a table of content and the content itself.

But if you scroll, the table of contents maintains his position sticky, how can I achieve this? Thanks!!

Here is my public share link: LINK
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