For instance, I work for the government and in their IT closet I see these big racks that are carrying multiple fiber lines which terminate into this box that says:
Nokia 1665 DMX which is apparently something called a “data multiplexer.”
I did some research but I still don’t quite understand what they do. Are they for combining data, tv, and phone signals together from the ISP?
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A multiplexer (and demultiplexer) is a device that combines multiple signals into one signal which can then later be split up again. And yes, it is a very vague term which can be used for a wide number of devices and software. I am not able to find any documentation I can understand on the device you have. But from what I can gather there are many slow speed inputs to it, probably one fibre from every floor, wing, department, or tenant in the building. There are then two high speed fibres from the unit that goes to a different building, maybe a datacenter or network central on the campus, base or business park. There a similar unit will split the signal into the same slow speed fibre cables. So you basically end up getting 10-20 individual network links through only two fibre cables.
The difference between a multiplexer and a network switch is that a multiplexer does not even look at the data. It will just pass the signal from one wire to another regardless of what the data is. A switch however, or even a router, will look at the data to find the destination address and send the signal in one of many different possible paths. If you think about this as a post office a multiplexer will just go around collecting out-trays into one big stack so it can be easily shipped to the post office. While a switch will look at the address on each envelope and then sort it into different inn-trays or send it to one of the neighbouring post offices.
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