Requesting assistance in creating an HTML page that features a dynamic drop-down menu. The drop-down should trigger a list to appear upon selection, which can be sourced from either a text file or plain HTML document.
The envisioned functionality involves choosing a product name from the initial drop-down menu, prompting a new drop-down menu populated with vendors subscribing to the selected product. Upon another selection, a subsequent list of related data replaces the existing content on the browser screen. I trust this description accurately conveys the desired behavior.