Drupal 8 Theme Customization [Are subthemes needed?]

Hello,

I recently started using Drupal 8 coming from previously using Drupal 7 for most of my wesbites. 

With Drupal 7 it was necessary to use sub-themes if you planned on making custom changes to a theme so that those changes would be preserved when updating the base them.

Is the same true for Drupal 8?

The Twig system is new to me and I am trying to make sure I do this right. Should I be using subthemes for the same reasons with Drupal 8 (saving custom changes)? 

Can someone point me to a good resource on how to set this up within Drupal  8 if it is indeed necessary? 

This page was a bit confusing:

https://www.drupal.org/docs/8/theming-drupal-8/creating-a-drupal-8-sub-t…

Mainly I just need to know what files should be placed in a sub-theme if necessary and where to make my changes within those. 

Drupal version: 


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