Intro Section with Dropdown Navigation

i can't seem to close the dropdown menu in the mobile design. putting an if else to check if the display is none or block doesn't work. also, is there other better way to design mobile responsiveness? it irks me how the design is fine in a pc browser (minimizing it and adjusting the zoom) but when i open the live site on the phone, most of it stays intact but some is off, like the sizes of image or the padding or margin. i only use notepad++ since it's lightweight and i don't know any frameworks (currently learning react) so i really only use basic html and css and vanilla js.