A data center uses multiple ISP Internet providers as a redundancy method to make sure that if one line goes down for the internet connection they can then re route the data and traffic through another ISP.
A data center will have multiple high speed communications lines coming in from the primary carriers such as Verizon, AT&T,and any others ISPS that are located in the area where the data center is located, these then connect to multiple primary routers in the data center.
And From there they will go to primary firewalls, then smaller routers, firewalls and switches.
A data center has people staffed 24/7 to make sure things are in working order and to fix servers and other parts of the data center in a timely manner.
They also have a monitoring room where they oversee the operations of the data center and can be notified right away should something go wrong.
Data centers also have armed guards on duty to protect entry into the data center since a lot of secure information is stored on the servers and not just websites.
Although websites should also be secured.
Here's a photo from the internet that shows a Data Center Control Room.
