Confused on how to approach this Layout

Hello. Hope you are having an amazing week.

PROJECT EXPLANATION

I was looking for website inspiration to learn new things and came across this site: https://www.wallofportfolios.in/portfolios/ayaneshu-bhardwaj/ and I wanted to give it a try but on a different setup.
Instead of having it as a sidebar, my navbar will be on top.
The content will be inside the GREY AREA and should have a fixed margin from both the top and bottom. exactly the same, so it accounts for the header height.

MY PROBLEM @_@

I just cannot seem to understand how to approach this layout. Tried using 100vh but either I am doing it wrongly or is not the right approach.
Content will stay inside the square, and will be scrollable INSIDE the square, but without scrollbars.
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