If I understand your question correctly, you'll need to run the HX installer, which collects the various IPs/network settings relevant to your specific networking environment, and deploys and configures the various HX nodes with ESXi
Probably a good place to start is the requirements page at: https://www.cisco.com/c/en/us/td/docs/hyperconverged_systems/HyperFlex_HX_DataPlatformSoftware/Installation_VMWare_ESXi/2_6/b_HyperFlexSystems_Installation_Guide_for_VMware_ESXi_2_6/b_HyperFlexSystems_GettingStartedGuide_2_6_chapter_010.html
Hyperflex OS will be ESXi or Hyper-V and storage controller deployed as virtual
HX installer will push config to the UCSM, ESXi to deploy HXDP on each node and finally create Hyperflex cluster
During installation VM network will be configured as well (you can also manually add vswitch after installation)
With a new HyperFlex cluster, it comes with an OS.
During the installation of a HyperFlex cluster with Fabric Interconnects, the network connection to the FIs are also configured.