Changes to a WordPress SAGE theme website

I need shown where to modify specific areas of a website I've been given to modify. I'm basic with WordPress and struggle figuring out "where" to modify a template. I have ZERO PHP experience so it's nearly impossible for me to know what file to modify (or widget, or plug-in, etc.)

I want to make some template layout changes to the HEADER area (create a video header instead of the default slideshow) as well as reduce the height of the header from 100vh to like 50vh and can't seem to locate which class file will let me do it. SAGE theme only allows ONE menu and it's the clunky dotted menu so it works for mobile. I want a dropdown across the very top but am just struggling how to make it mobile friendly without having to customize the code. I'd prefer that the simply WORDPRESS "MENU" area under the theme could just let me assign a top primary nav and add a footer menu. I make enough changes to the menu layout and adding primary pages that i want to just make the addition/change and it autoupdates top and bottom menus.

My willingness to hire this is based on having a tutorial as part of WHAT is being done to know how to easily find the areas in the code to change CSS and perhaps, remove things. I'm very used to raw files and seeing all the code in one page versus Wordpress using a gazillion includes to assemble a single page.