"With which" and "By which"

I want to question two sentences.

"Glaciation is the process by which the land is covered by glaciers."
I think this sentence are divided into two parts.
1) Glaciation is the process.
2) The land is covered by glaciers.
So I think If I wanna connect two sentences, the relatives are needed.
But, I don't know why does the first sentence need "relatives" because I think each sentence that means 1),2) sentences is full sentence.

2. Although some biologists attribute their survival to the rapidness with which they mature and their ability to breed large numbers of offspring during a single mating season.
This sentence is similar structure to The first question's sentence.
So I think "Mature" is intransitive verb, and it needs "with"
So I wanna ask the question : What kind of difference between "by which" and "with which"?
and How to use. Give some examples for me.