Where can I find custom content type templates file?
It's all in here Twig Template naming conventions

Nodes
Pattern: node--[type|nodeid]--[viewmode].html.twig
Base template: node.html.twig (base location: core/modules/node/templates/node.html.twig)

Theme hook suggestions are made based on these factors, listed from the most specific template to the least. Drupal will use the most specific template it finds:

1. node--nodeid--viewmode.html.twig
2. node--nodeid.html.twig
3. node--type--viewmode.html.twig
4. node--type.html.twig
5. node--viewmode.html.twig
6. node.html.twig
Note that underscores in a content type's machine name are replaced by hyphens.


The higher the number the less specific, For example, the last one (6) it affects everything. 

While (1) affects a specific node with a specific view mode, Ex: node--1--teaser.html.twig will only affect /node/1 with teaser view. 

For your case, you want to use (4), so in your case, its node--news.html.twig