How do I do namespacing in a module from an external project which is not in the src directory? Based on Why do I have conflicts in namespacing? I adjusted my complete system.

As long as I do namespacing in /src, it is goes all very well.
I included my project from Github. Those class files are located in modules/custom/tb/vendor/ccxt/ccxt/php.

When not working with namespaces I included them with include "vendor\ccxt\ccxt\ccxt.php";.

How should I include those classes to module? Because of the Composer structure, those files are in  the vendor sub-directory.

Because of namespacing in Drupal, use \Drupal\tb\&lt;classfile&gt; doesn't work.

The content of the composer.json file is the following.

    "name": "org/tb",
    "description": "This extension provides new commands for Drush.",
    "type": "drupal-drush",
    "authors": [
        {
            "name": "Author name",
            "email": "author@example.com"
        }
    ],
    "require": {
        "php": "&gt;=5.6.0",
        "arcturial/clickatell": "^3.0",
        "ccxt/ccxt": "^1.14"
    },
    "extra": {
        "drush": {
            "services": {
                "drush.services.yml": "^9"
            }
        }
    }
}
From your post, it looks like you added the above composer.json file, then ran composer from the module root. This is not the way to do it. This will create a vendor folder inside your module folder, which is not what you want.
What you need to do is add the composer.json file to your module, then push that to the remote repository. Then use Composer to install and manage the module. Doing this will add the library to the vendor folder for your project, and the library will be available when calling \ccxt\ccxt