Set the theme defaults for various settings. You already saw that you can set the WP standard settings for the Front and Blog page and the main menu in the General section of the theme’s option. You can go on in that very block and set more basic layout options (page layout, content color, background images…) and build and include several widget areas (footer, sidebars, slide in area…).
In the menu defaults you can set the look & feel of the menu. Basically you set the color schemes, paddings and the logo that shows. You can build different sets for the first appearance of the menu and the sticky one.
The default options for pages, posts and custom posts are divided into single(detail) view options (the single post or page display), Overview (behaviour on Standard Blog, Category, Archive Pages), the Page title and the Page Head/Hero Section. The Posts also have the featured media options additionally.