Here is a simple demo to build a static website with GitHub Pages.
To build a static website for your organization with custom domain name, you need to register domain into Pages settings of your organization:
https://github.com/organizations/<organization-name>/settings/pages
Then, you need to redirect your DNS to GitHub.
In example, a A record for your destination domain with theses values (see official documentation for updated values):
185.199.108.153
185.199.109.153
185.199.110.153
185.199.111.153
As you can show in current repository, I just put index.html page and that's all. You can add all your assets here, like any static website.
Got to https://github.com/<organization>/<repository>/settings/pages (Repository Pages settings) to configure your domain and HTTPS.
Enjoy!
Here is the static website from current repository: