I worked there for 3 years and yes GitHub development happens on github.com. Of course there’s ways to deploy and rollback changes while the site is down but that’s very unusual. The typical flow happens in github.com and uses the regular primitives everybody uses: prs, ci checks, etc.

The pipeline for deploying the monolith doesn’t happen in GitHub Actions though but in a service based in jenkins.

Fun fact: playbooks for incidents used to be hosted in GitHub too but we moved them after an incident that made impossible to access them while it lasted.

if GitHub Enterprise Server is anything to go by, they build (almost) everything for containers, and the entire site is hosted by containers managed by Nomad. So there are probably lots of older images around that they can fall back on if the latest image of any container causes problems.

How they would deploy the older container, I don't know.

A lot of this is guesswork, I don't work for them or anything. And I know that GHES in the way that my employer manages it is very unlike the way that GitHub host github.com, so everything i've assumed could be wrong.