I am looking to understand if Shopify Plus is required for a headless implementation. Note, I understand that headless can mean different things to different people. In my case, I mean a full site as opposed to a custom app to provide a single function/feature.
It seems as it is not required as all customers have access to the Storefront API. However, I can't find much or any information on it beyond what is provided in the Shopify Plus area of the website https://www.shopify.com/plus/solutions/headless-commerce
Are there bits and pieces of access that is different depending on Shopify vs Shopify plus?
No, in fact you can do Headless ecommerce with just Shopify Lite subscription ($9/mo) by utilizing Storefront API.
What Shopify Plus provides does not directly benefit headless ecommerce as much except for some additional cart/checkout manipulation with Shopify Scripts and also Checkout styling (if you redirect to Shopify checkout).
@agustin-atomic You will need to use Storefront API (GraphQL) mutations to manage authentication and other customer related functionality.
Their Storefront API library (JS Buy SDK) is pretty basic for retrieving products and managing cart, however, for this you will need to set up your own GraphQL client to make these calls.
You can find documentation here: https://shopify.dev/docs/storefront-api/reference/mutation/customercreate