This is a super simple fix. If you are trying to set up a centos 8 stream server with just the boot iso and the internet. You need to select the repo that it can pick from so you can get your packages. When you are selecting your isntallation source, the “on the network” source you can pick for a repository URL is:
mirror.centos.org/centos/8-stream/BaseOS/x86_64/os/
Select done and then you can select your packages. My go to is a minimal install, then you can continue your installation.