Hi @Txlon5,
Welcome and I will try to help explain.
What Netlify does
The Netlify hosting service does not run a NodeJS server. It is serving up your static site on their CDN (content delivery network). The service hosts static assets. Sites and applications access backend servers through API’s and Netlify offers Netlify Functions for added superpowers if you need them as a plugin.
But it is running NodeJS when there is a deploy
During the deploy process of a GitHub, Gitlab or Bitbucket connected site, Netlify creates a build environment running NodeJS so you can build your site during the deploy process. The site is built into a directory or location for the deploy to the Netlify CDN.
What does starting a server on deploy actually do?
When there is a server started and running on Netlify in the build container, it is doing nothing other than hanging up your build process waiting for a response that will never come. A server started during local development workflow allows the ability to test a site as if it was running on the CDN.
Can there be dynamic data in a Netlify hosted application?
Sure, but this data needs to be accessed through a fetch/request from the client’s browser using javascript. The application will make this call to an API on a remote server. It can be a third party service or you have the option of using Netlify Functions if you want to.
Hope this helps clear things up a little.