Banking software, hospital software, nuclear reactor software. Not all people are writing self-contained web applications.

Late reply, but yes, it's hospital software.

An EPOS application:

  PS D:\project\> (dir -include *.cs,*.cpp,*.h,*.idl,*.asmx -recurse | select-string .).Count
  1626667

At a previous employer, a piece of chip design software got to 1.25m lines of C++ over the course of a decade.

Even worse, probably one of those C++ enterprise software projects.

Well considering Visual Studio was mentioned enterprise C#/C++ project would be a safe bet.

video games codebases are sometimes this large