How does fibre actually carry data?
As pulses of light down a strand of extremely pure glass, guided by a physical effect that keeps the light from escaping even as the cable bends.
The physical mechanism. A fibre has a core of glass surrounded by cladding with a slightly lower refractive index. Light entering the core at a shallow enough angle hits the boundary and is totally internally reflected back in — not partially, but completely. It therefore travels the length of the fibre bouncing along inside the core, losing remarkably little energy.
How little. Modern fibre loses roughly a fifth of the light per kilometre. A pane of window glass that pure would let you see through several kilometres of it.
Single-mode and multi-mode. A multi-mode fibre has a wide core, allowing light to take many paths of different lengths, which smears pulses over distance — fine for a building, not for a city. Single-mode fibre has a core around nine micrometres across, so effectively one path exists and pulses stay sharp over hundreds of kilometres. Long-distance and most modern access networks are single-mode.
How one strand carries so much. This is the part that surprises people. Wavelength division multiplexing sends many independent signals down the same fibre at slightly different wavelengths — different colours, loosely speaking — separated at the far end by optical filters. Dozens or hundreds of channels share one strand, and capacity is upgraded by changing the equipment at each end rather than the glass in the ground, which is why fibre laid decades ago is still gaining capacity.
How it reaches houses. A passive optical network splits one fibre from the exchange between many premises using purely optical splitters with no powered equipment in between — cheap, reliable, and the reason fibre to the home became affordable. Capacity is shared, which is why contention exists on fibre too.
Why it still is not instant. Light in glass travels around two-thirds of its speed in vacuum, so distance still costs latency — and most latency actually comes from the equipment at each end.