ISPs or dark fiber providers have detailed maps aligning with road networks. If there’s a suspected fiber cut they’ll run an OTDR from the ODF box or the source photonic equipment which will show high reflection/ signal degradation where the break occurred to the nearest 100m dependant on test granularity.

I believe they use devices that act like some kind of radar. They send a signal across the wire and measure how long it takes to bounce back. They can get a really good estimation of the distance between the ping and the cut.

But maybe others can explain how this works.

Edit: this website explains other methods very well https://www.electricaltechnology.org/2015/06/cable-faults-ho...