Landing Page using Mixture of Grid and Flex to layout elements

Firstly do not set an explicit width on the body. You should create a container that stores everything inside of it, and you can give it a max-width. That way it is flexible but will never grow past the maximum width you have set. As of now, your code is completely non responsive firstly because of using the fixed width on the body. Secondly you should start by designing the mobile view first. This is simple because everything just stacks on top of each other. You don't need CSS grid for Mobile. Once you have the mobile design looking perfect, expand the screen until the mobile design starts to look a bit weird, this will be your first media query. Place a media query for this screen size and change the design elements inside this media query until it looks good again. You already understand CSS Grid by the looks of it, so this should be easy to do. That's pretty much it, just try and avoid setting explicit widths on anything if you can, you want the content to be flexible, use max-width instead.

Hello, I advise you to add semantics to your page. On the other hand, some people find it easier to start developing first for mobile and then for desktop, maybe that can help you.

You should also add break-point to make the respective changes between mobile and desktop. In my opinion, you can use Flex and Grid together, both are powerful tools.