In every IT company its not like only one person will be handling all application infrastructure requirements it will be distributed among multiple teams and all teams works based on requirements given to them.
If we take example of THREE TIER APPLICATION
Server Team - Create the VMs either windows or Linux OS and assign IPs to servers received from network team.
Network Team - Will create subnets, CIDR block, provide IPs to be used for servers.
Platform Team - Will setup Middleware web & App Setup with application configuration and deployment with application dependencies.
Security Team - Will perform vulnerability assessment, system audit and will check compliance.
Backup Team - Will start taking backups of critical files and paths once application goes live.
Monitoring Team - Will do timely monitoring of servers.