This may seem like an odd/broad question, but how does a server know not to render Express.js files and not to expose the content similar to how anyone can see a javascript file, and read the script being executed. Do node servers like Heroku protect them ? Sorry just new to express and node. Is it similar to how PHP syntax/scripts are hidden and protected in a Apache server?
It depends on the server configuration. On a poorly configured server, the .js files might be accessible.
With a nodejs/expressjs server you define a base folder that contains public files, e.g. public and files outside of that public folder are not visible, because the server doesn't serve them to the outside. If you configure the wrong directory, e.g. ., then the expressjs code files would be available to browsers and would be rendered as-is to them, potentially revealing unsafe data like configuration, passwords and so on. Since the default configuration and all code examples make sure that public is defined as the public folder, the risk of accidental misconfiguration is low.
If you run an apache httpd or other webserver on the same host, you have to make sure that the node application is not inside the webroot of any vhost, otherwise the files might also be visible, because to the apache httpd they also look like simple static files, ready to be sent as-is to the browser.
It is different from PHP files, at least in the case of apache httpd or nginx, because those are usually configured so that PHP files are files to be executed, not static files to be served to the outside. However, if the apache httpd or nginx doesn't know about PHP, either because it isn't installed or isn't configured, then PHP files inside the webroot would also be shown to the public as-is. Display of files for the apache httpd can be prevented using .htaccess files.