What does 现代化 ('modernize') mean in ...兴建了百万平方米的现代化厂房...?
Literally translated, 现代化 is "modernised". Actually I think the construction of Adj+化 in Chinese is actually mimicking the construction created by -ize in English.

Considering only the language aspect, I think 现代化 means the same as 现代(adj.) does. If there is any difference, 现代化 might implies a process of transforming something to be modern, hence "modernized". But in many of its modern usage it is does not. For example, I don't think the sentence in this question implies that those factories or plants were not "modern" but become "modern" by renovation or upgraded equipment. I feel they were built to modern and simply are modern.

This lead to my other point. I think 现代化 is overused in modern Chinese. You see 现代化 was a widely used propaganda term in the context of stimulating industrial and economy growth in the history of PRC. The proper term are "four modernisations", which basically equalised to "better" in the eyes of public. Nowadays, everything is already quite "modern", so there seldom be a "modernisation" process happened to an object described as 现代化的. Most of the time, they are just modern. However, we still see the world quite often, mostly in official speeches or promotional materials.

I'm not sure what is the best way in English that can capture this subtleness, and it's now become more or less a style thing. So in most of the cases, 现代化 = 现代 unless you want to give it an official or tone that relate people to that ideology.