While cramming all your code into a single source file is always possible (even for huge programs), a larger code base is far easier to manage if you organize related source code (functions, constants, types, and so on) into logical, composable units, each in their own files. Not least because you and others can then construct different applications out of these same building blocks. As of C++20, the composition unit of choice is a module.
The more your code base grows and/or the more third-party libraries ...
