Infrastructure as Code
Infrastructure as Code has two main sections to it. The first is running the code itself and executing the change onto the cloud platform. The second is maintaning the version control of the code. i.e multiple changes by multiple people to the code.
For executing the change one could use Ansible or Terraform.
From a high level you simply run an Ansible playbook while having changed the variables files to make changes to the environemnt. One could do this without getting too much into the way Ansible is working. Inside Ansible there are roles and tasks which divide the execution of the playbooks in a structured format so that writing complex playbooks is easier.
The second part regarding version control of the code is required because there are multiple people in a team making multiple changes to the Infra as Code variables. So for example one person could be adding a firewall rule subnet to one firewall and another person could be adding a firewall rule to another firewall. So if you imagine that all the firewall rule subnets are actually present in one variables file for all the firewalls then you need version control to coordinate these two changes to the file. This version control is done by Git and Bitbucket mostly and these are the two famous tools to maintain software code versioning.
This is definitely similar to what any large software system build and maintenance would require where multiple software developers are writing code and changing code in all sorts of code files at the same time so you need a version control system to maintain consistency. These have push and pull mechanism where when you make a change locally and push it onto the main file and at the master file you pull the change. It also has peer review mechanisms where other team members can review your code differences before they allow your code to enter the main repository.
To conclude, imagine you have 30 Azure VNETs (Network VRFs) and 30 Azure Firewalls in your product deployment. As people ask you regularly to make firewall rule changes and add and delete subnets it requires either manually going to the Azure web portal and making changes their or you could use Infrastructure as Code and make the changes via Ansible playbooks and git/bitbucket variable files.