I'm curious, do they use photodiodes or something? As far as I know, that would be the cheapest way to do high speed communications over fiber (fibre? I'm 'murican) but I'm not sure.
Yep, high speed photodiodes are the simplest way to do this. A receiver will usually consist of a photodiode and transimpedance amplifier (current in, voltage out). After that, it's just a high speed serial electrical signal, and that gets fed into clock data recovery circuitry, deserializers, etc.
There can be components in front of the photodiode, though. Wavelength division multiplexing is used to send multiple signals over one fiber. 40G and 100G Ethernet uses 4 parallel data streams, and you can get WDM modules that include the optical filters necessary to combine and split as needed. In this case, the receiver would consist of a WDM demultiplexer and four photodiodes and TIAs.
For coherent transmission, an interferometer is used to split the light out into in-phase and quadrature components, which are sent to two separate photodiodes and TIAs, followed by very high speed ADCs and a pile of digital signal processing.