It's definitely being routed over fibers, sure, but while faxes in theory can routed over the internet (T.38, https://en.wikipedia.org/wiki/T.38 https://en.wiki

It's definitely being routed over fibers, sure, but while faxes in theory can routed over the internet (T.38, https://en.wikipedia.org/wiki/T.38 https://en.wikipedia.org/wiki/T.38, which what Google Fiber is using https://support.google.com/fiber/answer/6377387 https://support.google.com/fiber/answer/6377387), in most POTS cases it's still done outside internet because of latency and compatibility concerns (unlike VoIP that human perception can mask small jitters), it's just transformed into light and back for long-distance calls (or if the machine at the end uses EPON T.38).