If you want layer 1 (physical location of dark fiber) you have to get the GIS dataset from the carriers. The data sets exist, but they're often protected as proprietary information or under NDA. I have a QGIS setup with a ton of stuff in it.

There are other ways of acquiring layer 1 data which are labor intensive and involve the equivalent of filing FOIAs for construction permits with local city and county agencies, etc.

Big facilities based ISPs that have a lot of fiber out there underground and aerial make extensive use of GIS software. Their construction groups will have their own full time GIS staff positions.

You'll have to follow the cable. Since this forwarding is entirely passive there's no way to know.