A big project in C. How to build a job so that it doesn't fill up with code?

Question

C++ / C#

Question for general development.
An acquaintance works in a project in pure C (in a team, of course). The number of lines is about 1,000,000 IMHO not very convenient for a pure C number of lines.
How would you work on such a project?
ZY At once I will make a reservation a question rather theoretical.

Answer 1

1) We divide into categories, each category - into its own folder. If necessary - more subfolders.
2) Transfer the common or old code to the library, include only .h + .lib
3) Use the version control system.
4) Introduce a limit on the number of lines per file, redistribute the code in such a way that there are no more than 1000 lines in one .c file (IMHO even 500 per s is already a lot.)

Answer 2

I would break the code into separate files depending on the functions. Well, Doxygen, of course.

Answer 3

Split by files. Use tags in function names.

Answer 4

OOP: functions, classes, libraries!

Answer 5

document using UML
not directly to the class and function, but at least the general skeleton and behavior.