How are large companies organized that one person cannot break the whole system in them?

Question

Business Informatics

How do they control the quality of the tasks performed by employees and prevent hacking of systems by their own employees in large IT companies?

Answer 1

If you are interested in what exactly they use and how they do it, then:
Active Directory (or LDAP) - look on the Internet for what and why they use it.
Vlan - to isolate users from each other or network segments.
Eventually. For mere mortals, when they log in using the login and password given to them, their user is limited according to GPO rules. For example, they can't run anything other than certain programs. Or maybe at least some.
Something like this. Although whatever you do from leaking information, nothing will help.