Can I by default turn on this feature for all my existing and for new user

This is a very common question. Unfortunately, the plugin still doesn’t have a dedicated settings page to configure/enforce the second factor authentication settings for all users. There is an open GitHub issue for adding a settings page.

There is also an open issue for enabling 2FA for all users which includes a temporary solution for doing that through a filter:

add_filter(
    'two_factor_enabled_providers_for_user',
    function( $providers ) {
        if ( empty( $providers ) && class_exists( 'Two_Factor_Email' ) ) {
            $providers[] = 'Two_Factor_Email';
        }

        return $providers;
    }
);

This enables the two-factor authentication via email for users that don’t have any second factor authentication configured.