Install multiple PHP version on Ubuntu for Virtualmin

 Enable ondrej/php repository

LC_ALL=C.UTF-8 add-apt-repository -y ppa:ondrej/php && apt-get update

Install additional PHP packages

apt-get install php*-{cgi,cli,fpm,pdo,gd,mbstring,mysqlnd,opcache,xml,zip} 

Note: php* should be replaced with a specific PHP version, i.e. php7.4, unless you really need to install all available additional PHP versions.

Configuring the second PHP Version in Virtualmin

Once you have completed the installation of a second PHP version on your server, you can verify that Virtualmin sees it by logging into Virtualmin, and clicking System Settings → Re-Check Config. You should see something like this:

The following PHP execution modes are available : fpm
The following PHP-FPM versions are available : 8.0.13 (php-fpm) 7.4.30 (php74-php-fpm) 8.2.0 (php82-php-fpm) 

You can configure which one is the default PHP version used on new Virtual Servers. The default is to use the newest available. You can change that default in System Settings → Server Templates → Default → PHP Options page.

Configuring Individual Virtual Servers

You can configure the PHP version being used for a specific Virtual Server by selecting Server Configuration → PHP Options.

Source: https://www.virtualmin.com/documentation/web/multiplephp/