How to fetch fields in a view in the overrided twig file?
You can do it like this:

{{ fields.[machine_name].content }}


The machine name is shown in your content type manage fields.

So it would be:

{{ fields.title.content }}
{{ fields.field_designation.content }}


Also, your template name needs to be: views-view-fields--employee-details--block-1.html.twig

Make sure you clear/flush the cache to see the changes.