How do I find and alter the Views field markup in Drupal 8?
First, enable twig debugging in sites/default/services.yml.

Then that should print a wrapper around each template that tells you which template it is exactly using for that piece of marking. In this case, it is probably views-field.html.twig or so.

Twig also reports what kind of suggestions you can use (e.g. a node type specific template for node.html.twig), but that doesn't work for views because views uses a different approach for that, and twig debug doesn't understand that yet.