Why does the margin move the parent?

As it was clear to me that the margin sets the indents inside the parent,

but why does the parent leave the indents down here?

Not sure I understand the question, but the child margin is collapsing through the parent element. In the next challenge, you add padding to the parent element to fix the collapse.

Another option is to set display: flow-root; on the parent element.

Yes, you understood everything correctly, but I don’t seem to understand why this is done, if the margin sets the padding for its element, then what does the parent have to do with it? In general, as always, it is impossible to understand))

You seem to have difficulty with English. You might find it easier to reference the Russian language translation of MDN: MDN Web Docs