It’s not mentioned but I’m assuming that they built their own OIDC/OAuth backend and not use existing ones (eg okta, Auth0 etc).
It would be interesting to know the details of how they’re doing authorization. It appears that it’s all or nothing but I might be mistaken.
Running an OAuth2 server isn't tremendously involved. There are good open-source projects like https://github.com/ory/hydra that are pretty easy to configure.