Install Remi Release repo and clear cache

yum -y install https://rpms.remirepo.net/enterprise/remi-release-7.rpm && yum clean all

Install PHP packages version 7.4 and/or 8.0

yum -y install php74-php-{cli,pdo,fpm,zip,gd,xml,mysqlnd,opcache,bcmath,imagick,mbstring}
yum -y install php80-php-{cli,pdo,fpm,zip,gd,xml,mysqlnd,opcache,bcmath,imagick,mbstring}

Configuring Individual Virtual Servers

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

The first line there specifies what PHP version will be used by default.

If you wish, you can specify a different PHP version to be used for a specific directory.

Src:
https://forum.virtualmin.com/t/php-7-4-not-showing-as-option-under-virtualmin/104088
https://stackoverflow.com/questions/64370480/linux-add-php-7-4-to-webmin-virtualmin
https://www.virtualmin.com/documentation/web/multiplephp/#Installing_PHP_74_andor_80_on_CentOS_7