Is it worth it after knowing the basics of css html to immediately learn and write on preprocessors?

Question

Preprocessors

In principle, now I began to understand that it makes sense for me to use pure css and html without preprocessors, because, as I understand it, they are used everywhere in companies, this is important for getting a job, since everywhere in vacancies one preprocessor is indicated. And how would you answer that question. Namely, am I not mistaken, or is it better to learn html css better first?, although I know the basic elements of html semantics, js basics, css (position, display, etc. except css3 but I think I can learn this using preprocessors.)
And I am also interested in one more question, and here I unfortunately do not quite understand everything, namely:Necessarily, vacancies sometimes indicate knowledge of bootstrap or foundation, please explain to me how to understand this, is it also important to know, or what exactly is so important in these libraries that they have become so third-party from the specification but so important?