Other comments give lots of examples of organizational complexity.

Here is an example of code complexity: Changing an object from enum to String is trivial to do in a small codebase. Making this small change in a live system like Youtube becomes a 6 month long migration with multiple SWEs working on it, when you have to do it in a codebase with 10s of millions of lines of code.

A lot of complexity comes from code size, multiple teams may end up creating duplicating products/services if there is no standardization etc.