If you serve on localhost you can usually access from other devices by using the "servers" ip address. So if your desktop where you're running the server has ip 192.168.1.10 then you can go to http://192.168.1.10 in the browser of another device on the same network.
But `localhost` is also an alias specifically for the loopback address (typically `127.0.0.1`), so "serve on localhost" can reasonably be interpreted as "serve on 127.0.0.1" which will only be available to other programs on that host, and not to others devices on the local network.
Also if your host and client devices both support mDNS / Bonjour, you don’t even need to type the IP address.
For example if your Ubuntu Linux desktop machine has host name foobar, and you run a http server on for example port 8000 then you can use your iPhone and other mDNS / Bonjour capable things to open
And likewise say you have a macOS laptop with hostname “amogus” and for example http server is listening on port 8081, you can navigate on other mDNS / Bonjour capable devices and machines to
