Layouts are loaded in a hierarcy:

block-beta
  columns 2

  block
    article
    component
    def["default"]
  end

  space

  block
    with["content-with-menu"]
    without["content-without-menu"]
  end

  space

  block
    base
  end

  block
    example
    json
  end

By overriding one of the base templates you modify how all templates extending it works and what content it has access to.