HTTPS Web Service and Backend Service on the same Server

Is there a way for to setup a backend service to listen on HTTPS on an already existing Web Service that is already serving on HTTPS?

I originally had both being served on HTTP, but recently added a CA for the Web Service. Both work perfectly fine on HTTP, but when trying to access the backend (from port 5000) from HTTPS, I get no response (I was originally serving the backend with PM2). I needed to update the ajax calls from the front end to call the backend on HTTPS instead of HTTP.

I looked into applying NGINX for the backend, but cannot find resources for my specific situation and I have no clue how to configure NGINX to listen on port 5000 for HTTPS (which doesn't make sense to me since HTTPS is port 443).

That's a Basic request, I will answer it in generic as the User did not provide enough Information to be more precise. REMINDER: So, this will answer it in Generally.

Pre-Infromations

User tells that he wants to listen for or to port 5000 for https

Standard http to https Redirect

server {
        server_name *.domain.tld domain.tld;
        #listen for Port 80
        listen 80;
        #redirect to https
        return 301 https://$host$request_uri; 
}

Listen NGINX on port 5000

server {
        server_name *.domain.tld domain.tld;
        #listen for Port 5000, http
        listen 5000;
        #add stuff here
}

Standard https Server Directive, using Certbot for Certificate Generation

server {
        #listen on 443 for SSL (https) with http 1.0/1.1
        #listen 443 ssl;
        
        #Listen on 44 for SSL (https) with http 2
        listen 443 ssl http2;
        
        #What we listen for domain      
        server_name *.domain.tld domain.tld;
                location / {
                root /var/www/vhosts/domain.tld;
}
    ssl_certificate /etc/letsencrypt/live/domain.tld/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/domain.tld/privkey.pem; # managed by Certbot
}

Basic Reverse-Proxy Listen on Port 80 http AND port 443 SSL

server {
        #listen on 80 for https with http 1.0/1.1
        listen 80;
        
        #listen on 80 for https with http 2 (disabled, certbot does not understand it on port 80)
        #listen 80 http2;

        #listen on 443 for SSL (https) with http 1.0/1.1
        #listen 443 ssl;
        
        #Listen on 44 for SSL (https) with http 2
        listen 443 ssl http2;
        
        #What we listen for domain      
        server_name *.domain.tld domain.tld;
                location / {
                        proxy_pass              http://127.0.0.1:5000;

                        #https even works
                        #proxy_pass             https://127.0.0.1:5000;                            
                        proxy_set_header        Host $http_host;
}
    ssl_certificate /etc/letsencrypt/live/domain.tld/fullchain.pem; # managed by 
    Certbot
    ssl_certificate_key /etc/letsencrypt/live/domain.tld/privkey.pem; # managed by 
    Certbot
}

Conclusions

If you want to reach your Backend on Port 5000 you have to use proxy_pass, if you want that NGINX listen on port 5000, in case its not been used listen listen 5000 ssl;

And Also, as others already said, SSL is a Protocol and is not bound to a specific port, its commenly used 443 but it must not listen to it, i also could use 61337 for it.

I'll update the Answer in case the OP is going to update the question