The foo.stackexchange.com names are subdomains, they all go to the same actual IP. For example:
$ for i in unix ubuntu apple workplace english french; do ping -c 1 $i.stackexchange.com | grep PING; done
PING unix.stackexchange.com (151.101.65.69) 56(84) bytes of data.
PING ubuntu.stackexchange.com (151.101.65.69) 56(84) bytes of data.
PING apple.stackexchange.com (151.101.65.69) 56(84) bytes of data.
PING workplace.stackexchange.com (151.101.193.69) 56(84) bytes of data.
PING english.stackexchange.com (151.101.129.69) 56(84) bytes of data.
PING french.stackexchange.com (151.101.65.69) 56(84) bytes of data.
You get the exact same response no matter what you put in front of the stackexchange.com:
$ for i in snotty snooty smelly grimy; do ping -c 1 $i.stackexchange.com | grep PING; done
PING snotty.stackexchange.com (151.101.193.69) 56(84) bytes of data.
PING snooty.stackexchange.com (151.101.193.69) 56(84) bytes of data.
PING smelly.stackexchange.com (151.101.1.69) 56(84) bytes of data.
PING grimy.stackexchange.com (151.101.1.69) 56(84) bytes of data.
There is no FreeBSD site on the stackexchange network, so you can't actually visit any site using the freebsd.stackexchange.com. Instead, you are taken to a landing page on the stackexchange.com parent domain:
[![enter image description here][1]][1]
This is a very common pattern. Most hosting providers, if not all, will give you multiple (even unlimited for higher end plans) foo.yoursite.com subdomains.
[1]: https://i.sstatic.net/ahfcL.png
