How to use custom block view mode in View?

Question

We have a custom block type block_countries and we are using a View(my_view_custom) to display the all the blocks where type = block_countries. Under block_countries view mode, we setup a "Full" view mode where we display certain information.

The view is displayed as a block and placed into a region (i.e. content region) from the theme. We want to style the block_countries "Full" view mode and setup a preprocess hook to do some processing, however we cannot find the way to do this.

We need help figuring out the proper twig template and preprocess_hook to call in order to get the desired out below. How can we do this?

Block type fields (block_countries):

1. body
2. language
3. country (Entity reference: taxonomy term)

Full view mode (fields shown):

1. body
2. country

View settings:

1. Format = unformatted list
2. show: custom block: Full (where full is the view mode)
3. Filter Criteria: custom block : block type = block_countries

We have tried the following but they do not work

Twig

views-view-unformatted--my-view-custom--block-1.html.twig

Hooks

mytheme_preprocess_views_view(&$vars)
mytheme_preprocess_views_view__my_view_custom(&$vars)

Desired output (per row returned from view):

<div data-country="{{ country }}"> {{ body }} </div>

Answer 1

Don't use custom blocks, use nodes or a custom entity type. Custom blocks are designed to be displayed as a block, not a view inside a block. They don't have a template on their own. See https://www.drupal.org/node/2704331.

Yes, it's not great, but it is unlikely that this will change any time soon.