Is this a correct way of using a literature review in a thesis?
Shouldn't I rather look at the reviews and then go directly to the
  sources[A] and derive valuable information in the way I interpret it?


Yes. You need to read anything that you cite, and you should read the original articles. 

But if the review articles were helpful to you, you should cite them too. For example, the review article [B] might compare techniques used in two of the original [A] papers. You might want to cite that comparison in [B].


  I have listed more than 30 mechanisms. If I were not to use these [B]
  articles I'd have to go trough 30+ [A] papers and basically recreate
  [B] papers and recreate a redundant literature review of said
  mechanisms that already exist in earlier papers.


Do use the review articles; they can help you and save you a lot of time. Without the review article, it would probably taken you a lot of time to figure out which [A] articles are relevant to your research, and to see how they relate to each other. Also, the people who write review articles tend to be experts in their field, so you're getting an expert opinion on what the state of the art is on a particular topic at the time the review was written.

You do still have to read the original articles, summarise them, and draw your own conclusions. And you should check for any important developments in the field that were published after the review article. But it's not nearly as much work as doing a literature review from scratch.