How were all of the modern pieces of infrastructure like gas lines, internet connections, electricity, water, sewers, etc...installed in cities, like London or Paris, that existed well before any of those were common let alone invented? And how were they put in place without ripping up the whole city laying down pipes and cables?
If you want a source for london see London Under London: A Subterranean Guide.
They ripped up the roads/pavements and layed down pipes and cables. Sometimes those pipes exploded.
Water and sewers have been around in at least an ad hoc form for rather longer than those cities have existed whith new supplies being added over the centuries. Southampton for example gained its first documented water supply in the 14th century (built by the local Franciscan Friars).
In britain the situation was largely regularised in the 19th century with various goverment bodies being formed to sort out the sewerage systems in particular. Indeed the history of local goverment in this period is heavily driven by the need to sort out the sewers and water supply. The Public Health Act 1848 created local boards of health which while in theory were meant to sort out the various outbreaks Cholera in practice also got involved in the ongoing reform of local goverment.
Still enough of the politics. Lets move on to the London sewerage system. Again roads were ripped up to build it although the system also redirects a number of streams into itself and used land recovered from the forshore of the Thames as part of the construction of the Thames Embankment for one of its main interceptor sewers (bigger sewers that other sewers flow into. they are egg shaped).
This brings us to exploding pipes. You see London (and a few other cities) had a hydraulic power network from the late 19th century. Think very high pressure water that could be used to drive machinery. Unfortunately the technology of the day wasn't quite up to it which resulted in bits of the pavement exploding from time to time.
Gas was installed in a rather piecemeal manner again by digging up roads and pavements. One thing they did make some use of was canals of all things. Canals were rather convient since they often ran between important places, only had one landowner and by this point where in such financial trouble that they were happy for someone to pay to dig up a bit of their towpath to put some pipes underneath it (although for some reason the ones in stroud were in places laid above the surface).
Electricty and telephone much the same although that could be put on poles rather than being burried under the street.
Fiber optic is getting us a bit close to the 20 year rule. Again much the same as gas and electric although it tended to use canals for much longer runs. Canals were by this point in goverment ownership and goverments were generaly delighted to find that someone wanted pay to dig up the tow path and leave it in a better state than they found it (this wasn't difficult). The other thing they took advantage of was that the London Hydraulic Power Company had gone out of business in the 70s. One thing it owned was the old tower subway (the second tunnel built under the thames) which amoung other things now contains fiber optic cables.