Weirdly, the CentOS 8 network installer does not detect mirrors automatically. No worries, you can just specify it manually in the installer and everything will “just work” after that.
http://mirror.centos.org/centos/8/BaseOS/x86_64/os/
Oh, another thing. When selecting a destination disk to write to, every disk with installer files/partitions are detected on are removed from the available destination disks. So if you have an old installer on the destination disk, you need to wipe or else it won’t show up as an available option.