Colocation facilities are interesting (I've been in a lot of them) and they generally (physically) fall in two buckets:
1) Repurposing of old telecom (and even telegraph!) buildings. CoreSite (a colocation provider) has a few floors (last I looked) in The Telegraph Building in Chicago[0]. If you look at the pictures of the exterior of the building it still says Western Union on the side! There are some interesting aspects of these - most of them are located in downtown areas. If you start looking around in major cities you will start to notice gigantic nondescript buildings with minimal signage, almost no foot traffic, and almost no (if any) windows[1]. If they have any signage at all it will typically be "Something Bell", Western Union, AT&T, etc. These facilities are interesting for a variety of reasons. Because they are repurposed telegraph and telecom buildings going back to the days of mechanical telecom switching[2] and DC battery banks they have floor load ratings that are just incredible. Something like 20x or whatever of a typical high-rise. Downside of these facilities is that on a square footage basis they typically can't support the cooling (and often electrical) requirements of modern compute equipment which is much more dense than the extremely heavy but relatively cool/low power mechanical switches and other equipment from decades ago. If you look at the picture of the Telegraph Building you will notice it's literally right next to the Chicago Stock Exchange. Needless to say these facilities are really popular for HFT stuff.
2) Purpose built facilities. These are typically located near population centers (for access to peering and other fiber) but out of downtown areas. Examples here are CH7 and CH3 from Equinix[3]. These are still close enough to major peering points and fiber distribution so access and latency isn't terrible but being in more "suburban" areas. Here they have more space to build, real estate is cheaper, and interestingly since 9/11 there has been a movement of sorts to move major hosting facilities outside of densely populated areas/potential terrorist targets.
Then for your Google, Facebook, NSA, etc of the world they can build in fairly remote (rural) areas where they have essentially unlimited space and significantly higher security[4]. The kind of thing where law enforcement and private security can setup perimeters where if you get anywhere near the actual hosting facility you better have a reason to be there - as opposed to a downtown facility where thousands of people walk and drive by everyday without ever knowing it's there.
[0] - http://wikimapia.org/30789188/Telegraph-Building-CoreSite-Da...
[1] - https://nypost.com/2018/06/26/nsa-using-att-data-center-in-h...
[2] - https://www.youtube.com/watch?v=xZePwin92cI
[3] - https://www.equinix.com/data-centers/americas-colocation/uni...