Layout and Bem. What is what?

Question

css

Help deal with, almost Bem'om. And so, there is this html code:

expand

<div class="Layout">

  <div class="Layout__row Layout__rowHeader">
    <div class="Layout__cellHeader">
      Пусть это HEADER
    </div>
  </div>

  <div class="Layout__row Layout__rowNavbar">
    <div class="Layout__cellNavbar">
      А это пусть NAVBAR
    </div>
  </div>

  <div class="Layout__row Layout__rowBody">

    <div class="Layout__cellBody">

      <main class="Layout__colMain">
        main
      </main>

      <aside class="Layout__colSide">
        side
      </aside>
    </div>
  </div>

  <div class="Layout__row Layout__rowFooter">
    <div class="Layout__cellFooter">
      Это FOOTER
    </div>
  </div>

</div>

For example, Layout_row and Layout_rowHeader are the same CSS file layout.css.

How correct is it if Layout__rowHeader is moved from layout.css to header.css?
Layout__row Layout__rowHeader
do this:
Layout__row Header__row

In general, to separate everything into layers, is that good?

How correct is it to typeset like this: https://codepen.io/git2ss/full/jOMwKWz
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