You are exactly right about the containers you see on the outside of the buildings. The inside of those buildings are designated at a level of cleanliness that most equipment can't meet or equipment like UPS units and the switchgear that have a potential to explode are kept outside of the buildings. The servers inside need to be protected at all cost. These places are designed with redundant on redundant for everything. Outages of equipment are scheduled well in advance and there are many blackout dates when maintenance is absolutely never to be scheduled on mostly relating to shopping days both brick and mortar and e-commerce as well as sporting events like the Super Bowl.

Although most data centers do have a particular end customer only many of them are setup to be shared. So some are strictly for Amazon, Microsoft and such while other will have 12 smaller end customers who move their service in and out of them.