Responsive Mobile-first Dropdown Nav with HTML/CSS/JS

Moving the side navigation in mobile view to the top of the page for desktop sizes was difficult. The page has to be refreshed when resizing the window to see the side navigation on mobile screen sizes. Also couldn't figure out how to make the dropdown menus (even when using the z-index property) appear on top of the h1 on desktop sizes. If anyone knows how to get this part working, feel free to mention it!

Thanks for any feedback :)