Qamimi entities to set the basic structure of the page in BEM?

Question

css

Hello.
There is a rather complex adaptive page in its structure, various flex blocks that are not content, they only set the structure, placeholders for future content: header, footer, sidebar, auxiliary wrappers above them, which help create a complex adaptive frame. Should these blocks be set as blocks or elements? For example, a header, which is located in a common block with a side menu. Both the header and the menu are independent logical blocks. But their styles are such that they will only have the look they have in that context. Although if you take them out of context, they will adapt to a different context in accordance with their styles, although they will look different. I'm a little confused. Blocks or elements for structure?

Answer 1

https://jsfiddle.net/fj4a0ap1/