What does it really mean to scale a web app and how is it done?

I'm knowledgeable in how to create a web application, using databases, front end and back end programming and such. I've never really needed to know how to scale the applications that I've created because they don't get many hits.

It eludes me how to scale up to the size of a facebook or google service.

From my understanding you have to have more machines capable of processing all the requests because a single one can't do it quick enough alone. Now I'm not sure how many servers google has but I know they have A LOT. I don't get it though, source code is written once and the same code is placed on all of the machines and every time there is a change to the source it has to be updated on all of the machines?

How do we deal with this complexity...