Poudriere multi mysql/php builds

[Doug McIntyre]

I notice that pkg.freebsd.org has packages available for
multiple mysql-server and php all available at once.

eg.
php56-5.6.30                   PHP Scripting Language
php70-7.0.19                   PHP Scripting Language
mysql56-server-5.6.36          Multithreaded SQL database (server)
mysql57-server-5.7.18          Multithreaded SQL database (server)
mysql80-server-8.0.0_4         Multithreaded SQL database (server)

I have a Poudriere system setup, and I know I can do a global set on
which MySQL or PHP to build and use by default, and I've also setup
different jails to build different combinations of what I want. 

But how does pkg.freebsd.org do it to offer it all in one spot?
I would like to do that internally as well.