How to get the field collection field value in twig file?
Say you have a field collection field_test and the field collection has fields say field_name and field_age, you can access them using the below snippet.

{% for details in node.field_test %}
    Name: {{ details.getFieldCollectionItem().field_name.value }}
    Age: {{ details.getFieldCollectionItem().field_age.value }}
{% endfor %}


See reference and 
another reference

I assume getFieldCollectionItem().field_name.value is what you are looking for.