The Theme Defaults will be used throughout the posts and pages unless you decide to vary from this standards.
All defaults can be altered on every single post/page.
The general layout for example is (by standard) available at the top right corner.
If you are editing a blog post you will additionally have the option to set a post format for that post (a little down that right column (standard order)).
The single page and post options appear underneath (standard order) the WYSIWIG editor / Visual Composer editor.
You will find all theme defaults here again. After switching from “Default” to “Custom” of the needed tab you will find a set of options that wont get the theme defaults but the settings you see now.
You will partly find more options (especially in the header and featured media areas) since these are things that you cannot define as defaults per definition. They are too special as that they could be defined as “default”.