Google has at least 8 data centers in the USA alone and some more Data Centers in other countries.
When it comes to keeping Google up and running and reliable they can do that because of the many data centers spread around the USA and the world.
Googles Data as well as Youtube Data are stored in many data centers across the globe and there's more than one copy of that data on the servers in other data centers.
This is to ensure they don't lose any data in the event of a hard drive crash or server crash or if one data center is destroyed they can still stay operational.
If you upload a video to Youtube it gets stored on more than one server and is copied to more than one data center so that when someone views your video it is at a data center closest to them.
That way they get the fastest video delivery from the Youtube servers.
It's also to ensure that your videos are not lost in the event of a server crash etc.
Google has all it's own data such as search engine data etc also copied on other servers as well as your photos etc that you upload to a google server.