mysqlnd with php7.3 on debian buster I ma quite new to drupal . I've tried to install commerce 2.x on debian 10. However, php-mysqlnd is only a virtual package referring to php-mysql. there is no real mysqlnd and commerce gets stuck with this dependency. 

Does that mean I cannot rund commerce 2.x on debian 10?
After taking a look at the php-mysql package for Buster it appears that the file is included with that package at /usr/share/php7.3-mysql/mysql/mysqlnd.ini.
I'd say that installing the virtual package should net you the module / driver you need. You'll likely need to enable the mysqlnd module and restart your FPM pools.
You can verify that the extension is loaded by
php -v # to get the version
php -i | grep mysqlnd

You'll probably want to enable it for cli as well as FPM.
Good luck!