How this web site was set up
Dave Schweisguth registered fix26.org and serves DNS for that domain from his personal server.
fix26.org resolves to Dave's web server, which redirects that address to www.fix26.org.
fix26.org also has a DNS TXT record which proves to Google that the same person owns it who has Dave's Google account. That was generated in Google Search Console ― not in G Suite admin, which is where the Google Sites help sends you, or Webmaster Central, which is where the Custom URLs dialog in Google Sites settings sends you!
www.fix26.org is a CNAME for ghs.googlehosted.com. (Google does not document this well and it took a while to find.)
The fix26sf Google Site is configured to have www.fix26.org as its canonical "Web Address".
Much of the above is documented here.