When using the ExternalProject_Add function, CMake typically provides built-in support for GIT , SVN , CVS , and direct URL downloads. However, if your project requires a proprietary or unsupported retrieval method, DOWNLOAD_COMMAND serves as a "catch-all" override.
The DOWNLOAD_COMMAND in CMake is a powerful configuration option used within the module to override the default methods for retrieving external source code. By specifying a custom command, developers can integrate non-standard version control systems, handle complex authentication, or perform pre-processing during the download phase. Core Functionality download_command cmake