Just like Nic mentioned, the Confluence port number should be configured on the server.xml file which is located on the Install-Directory/conf folder. You can use the below documentation for instructions on how to edit the server.xml file and configure a new port number for Confluence:
It depends on your installation, but generally you need to look at the server.xml that the Tomcat uses.
Confluence's default port is not 80 though, the installers tend to put it on 8090, so you've probably already changed it once and we don't know how (via reverse proxy, editing Tomcat server.xml, running a different application server, etc)
It's worth noting that you can change the port during initial installation (as I recall it, you're asked if you want the defaults or custom ports) - but in my recent experience going from 5.9.10 to 5.10, the port was reset to defaults without asking. Took me a few minutes to work out why it wasn't starting and answering requests.
It does seem to indicate that putting it behind a reverse proxy would be wise, even for internal access.
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