"The most extensive urban complex of the pre-industrial world." I'd hyphenate pre-industrial as industrial is the real word and you are forcing two vowels together which makes it look weird. It refers to a period before industralisation which is generally thought of as 1760 but could be different. This is the largest urban grouping from that period. I don't think you need "complex" in the sentence, it tends to mean things that are put together to carry out functions and that is also part of the definition of "urban" so I suspect it is a tautology. A bit like "icy ice". :-)

Out of all the urban (=related to the city/characterised by city life) complexes (I am guessing that they refer to a collection of buildings as it makes the most sense), the one urban complex that is mentioned is the most extensive (I assume in terms of number/amount of buildings) urban complex known before (=pre) the industrial world (=can be interpreted as "period/era/time") emerged.