Need help understanding the websites layout semantics

When making websites is the content area such as the text, background image and all content thats in the middle of the website (not the nav section) when you first arrive, is all that stuff within the header semantic tag? or is that part of the <main <section> area. And if its part of the main section area do they just have the background image expanded to reach the <header> tag Ex: min-height: 180vh so it covers the section and header