The basic idea of the <main>
element is that the content within it is considered unique to the document (which lends itself to the entire concept of individual documents within a site).
Since site-wide navigation is supposed to exist across the whole site, it should exist outside of the <main>
element.
Likewise for any content that pertains to the site as a whole, rather than being document-specific, such as sidebars.
To be clear, as Kunaal Topraniu mentions, you can place a <nav>
within a <main>
provided that it consists of navigation that is specific to the <main>
content, such as a table of contents. Site-wide navigation, of course, is not content-specific, and therefore does not belong in a <main>
element.