API for Magento -- 3

We need to use Magento v2 REST API to add products into Magento’s shopping cart from a remote website (not magento). So, the purpose is to build an API standing in the middle of the remote website and Magento. Every time a customer clicks a Buy Now button in remote website, a post is sent to the API with the product and customer information. The api connects to magento and: 1) if the product exists in Magento website, just add it to the cart. 2) If the product doesn't exist, add the product to Magento and then add to cart. The checkout process will be in Magento, so the customer needs to be recognized after being redirected from website A to website B to checkout. We also need the API to have an endpoint to add customers from website A admin and another endpoint to add products from website A admin. The API has to be made in Node.js In order to ensure that you have read this description thorough, please include the word 'migration' in the body of your response to this posting. All others will be ignored.