@geoffrey1900 things are a little more complicated than what @charliea21 describes, but the essence is correct. There are a few layers that a request to a specific Glitch url goes through, but basically a request comes in for port 80 or 443 (depending on whether HTTP or HTTPS) and it gets forwarded through our infrastructure and eventually ends up at port 3000 (usually) in your project’s container. We do only open a single port, but you could try something like what’s discussed in What's the port range users have at Glitch? to forward other internal ports.
Basically. you can open any port you’re able to access within the project container (since you’re not root that’s basically anything over 1024), but none of them will get traffic from outside the container (i.e. the internet) - that will only come into the single port that’s open.