Altering fields before rendering: hook or template?
You can configure views in many ways and the hooks, that are called, will change accordingly.

You can easily debug this, if you put a "catch all" hook into your module and print out the hooks that are called:

function mymodule_preprocess(&amp;$variables, $hook) {
   echo "&lt;em&gt;$hook&lt;/em&gt;&lt;br&gt;\n";
}


Then your page will be full of hooks, that are called at different places and you can try to narrow it down to the hook you need.

The next step would be to install devel and enable kint and debug the variables of the hooks, that you are interested in:

function mymodule_preprocess(&amp;$variables, $hook) {
   if ( $hook == 'views_view_grid' ) {
      kint($variables);
   }
}