How is this person creating a comma delimited list for collection items without custom code? https://webflow.com/made-in-webflow/website/Multiple-nested-Collections-with-unlimited-Items-on-single-pages-in-Webflow
according to the description, they are using the JQuery .load() function to grab the HTML data.
But if you mean how are they placing the commas, like you may have already discovered they are using the CSS nth-child pseudo class, is my guess - haven't dug into it too much.