CIs are simply any component that needs to be managed in order to deliver an IT service. A server, a virtual server, or even the configuration of an application could be considered a CI, for example
https://www.bmc.com/blogs/itil-asset-configuration-management/#:~:text=among%20your%20CIs-,Configuration%20items%20(CIs),considered%20a%20CI%2C%20for%20example.