Вывести конкретное доп. поле:
{% if collection.fields.banner.value %}
{{ collection.fields.banner.title }}
{{ collection.fields.banner.value }}
{% endif %}
Вывести все доп. поля:
{% for collection_field in collection.fields %}
<div class="field-title is-{{ collection_field.handle }}">
{{ collection_field.title }}
</div>
<div class="field-value">
{{ collection_field.value }}
</div>
{% endfor %}