What software does the forum use?

I would like to know about the software the forum is based on. Is it open source?
Can I use it in my current project?
Does anybody know?

It’s Discourse. I don’t know anything about it, but you can look it up on their website: https://www.discourse.org/

Oooh - I thank you @A_A for the quick answer…

I am admin of humane tech community, also based on Discourse. Personal opinion: best forum software to be found. It is OSS, you can self-host for free, or cloud-hosted paid subscription with full support (very responsive, they are on meta.discourse.org).