Thank you Mark! That is really helpful . I did have to change the owner and group of the parent folder to the code folder ('/var/www/moodle') being '/var/www' to 'www-data' to stop a composer install write permission issue. So then ran:
sudo -u www-data composer install --no-plugins --no-scripts
sudo -u www-data php admin/tool/phpunit/cli/init.php
That just leaves the oddity with the install docs! Over having '/var/www/moodle' as 'root' or 'www-data', so posted: https://moodle.org/mod/forum/discuss.php?d=412367 - as a separate question pertaining to that area.