Where do the values ​​in this formula come from?

Question

css

Hello! In the example for this article on this article on Habré , there is such a formula that interested me for calculating the font size: font-size: calc( 12px + (20 - 12) * ((100vw - 400px) / ( 1200 - 400))). Tell me where is she from? Where do the values ​​come from? And where can you read about it?
Here is an example

Answer 1

https://www.smashingmagazine.com/2016/05/fluid-type...

Answer 2

It's only strange why so many calculations, if all this is reduced to this form
Font-size: calc(1vw + 8px)