When computers were mostly standalone systems, authentication (checking people are who they say they are) and authorization (allowing them access to specific information) codes, along with databases containing user information, were self-contained on the device. Even in the early days of the web, sites would handle security independently, using custom and proprietary code.
Fast-forward to today and you can use the same login information across multiple apps and sites, either social logins or your custom enterprise login information for work. This is due to identity industry standards being widely employed across the web.
Identity industry standards are open specifications and protocols providing explicit guidance on how to design authentication and authorization systems to manage identity, move personal data securely, and decide who can access applications and data, so multiple parties can achieve interoperability easily.