most large corps have many networks, most firewalled off from each other and several removes from the outside world.
sensitive data in HR, new projects, financial data would be examples of where separate networks are used. Roles determine who gets access to which network and to which resources on a network. pretty sure its been like this for decades.
hackers do find ways to compromise firewalls and gain credentials some of which you probably can guess..
Where I worked it was all one unclassified intranet, with stuff like HR protected by password access. Prior to about 13 years ago they had over 40 disparate and incompatible unclassified networks. Then in 2005 everything was consolidated into SAP enterprise. Painful process.
My company managed to lose my personal info in a stolen unencrypted laptop, then my government delivered my SF86 to the Chinese (OPM "hack"). Information security is a unicorn.