How to display data based on an array of the second level of nesting?

Question

Vue.js

I'm confused, how to display the content only if a subsection is selected? that is, an array of the second level of nesting.
And for some reason, subsections do not work correctly for me, I don’t understand why. Need help.
SANDBOX

Answer 1

Obviously, we need to add one more property that will be responsible for the selected subsection.
https://jsfiddle.net/w0vrxcyg/2/