How google get the 'use over time for' data from the before internet years?
I am not sure if you are familiar with this, but in linguistics there is a term "corpus". It is a large collection of texts used for statisical analysis and other linguistic research, both historical and contemporary. 

There are corpora of digitalized texts, both literature and other domains such as newspapers, bulletins, etc etc. I recon Google uses data from such corpora.

For example: https://www.sketchengine.eu for English.

Here's a somewhat robust list of corpora: https://en.wikipedia.org/wiki/List_of_text_corpora