I wouldn't say [1] goes into any particular detail.
Nor does [2]. It says it can happen.
1) A population of virus or biotics has genetic diversity and mutations through generations when reproducing.
2) Some pressure is put on its survival (antibiotics, vaccines).
3) Those in the population that survive better (with the new pressure introduced) proliferate more than those in the population that didn't survive or thrive as much.
4) You end up with a population more resistant or immune to the chosen selective pressure initially used on it.