Assign a specific Shopify collection to a product for a custom breadcrumb using the collection handle and a metafield

product.liquid

{% assign productCollections = product.collections | map:'handle' %}
{% if productCollections contains product.metafields.my_fields.breadcrumb %}
  <a href="/collections/{{product.metafields.my_fields.breadcrumb}}">{{collections[product.metafields.my_fields.breadcrumb].title}}</a>
{%- endif -%}