Forums and an err or ur-

Layerpoint's Best Free Forum Software 2017, apart from including more platform software like UseBB Forum, Codoforum, YetAnotherForum, and IceBB Forum, provides some more detail in the mechanics of this board software. Here and in Wikipedia's comparison of Internet forum software, we see how they are powered.
phpBB and a number of others are released under the GNU General Public License which means you don't need their permission to change code. bbpress we are informed, is one of the popular WordPress forum plugins.
Discourse is self-hosted forum software. Discuss is written in Ruby - on - Rails

Other forums are written in PHP and have MySQL database as a backend.
YaBB forum is  written in Perl. I note Java and Javascript in the Wikipedia tables.

 All of which is to say you need a degree in information technology to decipher any of that.

II

The other aspect to note is that of an open source forum which implies an ability not just to post but to alter code. I could be talking out of my hat.