web-app using nginx and node - which is the web-server?

I have a web-application using nginx as a reverse-proxy and using the express framework as my backend in node.js. I am confused which is the web server, maybe both or maybe it's a grey-zone in my case. I use react, so the application features client side rendering, and nginx holds these files should it make a difference.

according to developer.mozilla.org

On the software side, a web server includes several parts that control how web users access hosted files, at minimum an HTTP server. An HTTP server is a piece of software that understands URLs (web addresses) and HTTP (the protocol your browser uses to view webpages). It can be accessed through the domain names (like mozilla.org) of websites it stores, and delivers their content to the end-user's device.

&

A web server first has to store the website's files, namely all HTML documents and their related assets, including images, CSS stylesheets, JavaScript files, fonts, and videos.

Taking this into consideration, I would say that Nginx is the web-server since it holds the html file. However, I really am not sure. Is it one of the two, both or is it a grey-zone?