Crystal,
The core wordpress engine is the CMS, but when that gets updated, the Themes do not get updated along with it. So if you build on the Twenty Fifteen theme, next time WordPress updates you would still run your site on the same Theme.
Sometimes (separately from the WordPress core updates) Themes are also updated by the people who create them. At that point you can decide if you need to update the theme or not.
Of course the Themes run on WordPress, so if you are going to update the core CMS, then you probably want to keep your Themes up to date as well, or you might run into some compatibility issues.
A child theme will not work in the absence of it’s parent theme. The child theme is just a collection of modifications to the parent theme that are managed separately so that you don’t have to tinker with the core theme itself.
I hope this helps.