Why my Nginx proxy succeed to find my node webserver since my docker-compose doesn't expose any webserver port on the network?

My node webserver uses express and listen on port 5500.

My dockerfile-compose doesn't expose any port of my node webserver (named webserver) as following:

​

version: "3"

services:
 webserver:
  build: ./server
 form:
  build: ./ui
  ports:
   - "6800:80"
  networks:
      - backend  // i let the backend network just for legacy but in fact webserver isn't in this networks
  command: [nginx-debug, '-g', 'daemon off;']
networks:
 backend:

​

My Nginx reverse proxy as following:

​

/request { 
proxy_pass http://webserver:5500/request 
} 

expectation: y request have to fail because of the absence of shared network between the two services.

result : request succeed.

​

I can't understand why ? maybe the default network between the container make the job ?

more info: the request fails when the reverse_proxy redirect to a bad port, but succeed if the domain name is wrong and the port is good as following :

proxy_pass http://webver:5500/request  > succeed 

I can't understand the Nginx / Docker flow here. Someone would please explain us what happens here ?