Projects are stored in .sublime-project files.
The project structure has JSON syntax:

{
    "folders":
    [
        {
            "path": "src",
            "folder_exclude_patterns": ["backup"],
            "follow_symlinks": true,
        },
        {
            "path": "docs",
            "name": "Documentation",
            "file_exclude_patterns": ["*.css"]
        }
    ],
    "settings":
    {
        "tab_size": 8
    },
    "build_systems":
    [
        {
            "name": "List",
            "shell_cmd": "ls -l"
        }
    ]
}

source: https://www.sublimetext.com/docs/3/projects.html