The default PHP interpreter for CiviHosting today is PHP 7.2. But that is only for web service, not for CLI. For CLI, the default version is 7.2.
All servers have PHP 7.1, 7.2, 7.3 and 7.4 also available for CLI of course. To run PHP 7.1, use:
php71.cli
To run PHP 7.2, use:
php7.cli
To run PHP 7.3, use:
php73.cli
To run PHP 7.4, use:
php74.cli
To run drush with a different version of PHP, use the full path for drush after the PHP executable, meaning something like this:
php73.cli /usr/local/bin/drush