Thank you, mithrila and merlock,
Those responses were very informative, even though I no longer think I need to have my Desktop computer even momentarily disconnected from the Internet using:
ip link set x down
… as you have kindly shown me.
I realise that it is likely that the following may not be of much interest to this discussion, however:
My (possibly misguided) motivation was to be able to make a new minimal installation of Manjaro on a disk where Manjaro is already installed.
That minimal Manjaro Linux installation was to run a trial Drupal (https://drupal.org) content management server without all the usual programs for Desktop use - firefox, chromium, kate, gnumeric, gimp, inkscape, etc. etc.
I had (naively?) hoped that if I used a manjaro-xfce-21.04-210506-linux310.iso with no connection to the Internet, I would be more likely to get such a minimal installation, to which I would then add Drupal (version 9), Apache, PHP and MariaDB or Percona (https://www.percona.com/) as a replacement for Mysql.
However, that initial Manjaro installation made without an Internet connection seemed unworkable. So I overwrote that installation by again installing manjaro-xfce-21.04-210506-linux310.iso, but this time with the Desktop Computer connected.
Possibly I should be using BSD, of which I have little understanding, or some other Linux distro which may be more focused on working as a server, if there are such distros.
I am still some way off getting my Drupal CMS to work.