how to fix container % and menu location

When i zoom in and out my website becomes thrown off because the background is full-width but my container and right menu stay a fixed number of px

Now, i barely know code, and i cant seem to find the php file that controls this [cause as we know nothing is really in the style file] it i’m hoping for css that makes everything a proportion [eg. 80%] rather than fixed pixel quantity [1500 px]
so that no matter what the resolution it looks correct.
also, oddly, i have buttons on one screen that i align in a row, but at one zoom level, and only that one–not the zoom above or below it– it cramps the container and the lat button goes to the second line. I’m hoping this issue would go away however when the container is a fixed ratio

additionally, does anyone know if I could have my right menu justified to the right of the screen rather than necessarily the side of the main container?

thank you!!