How to add meta tag to wordpress posts filter?

For example, I have an url:

mysite.com/author/admin

It’s not considered as single post or single page to add meta tag via SEO plugin. And to this page I want to add something like:

<meta name="description" content="All posts by author admin."/>

Is there are any way to do this? Thanks in advance!

There is not a standerized way to add HTML meta tags in WordPress but you can use wp_head action as a generic way to add meta tags.

I think description meta tag should not be in the theme, as you suggest in your answer, because description meta tag is a property of the document, nothing to do with the look and feel of the document, that is, the theme.

This is a samlple code to do it:

add_action( 'wp_head', 'cyb_author_archive_meta_desc' );
function cyb_author_archive_meta_desc() {

   // Check is we are in author archive
   // https://developer.wordpress.org/reference/functions/is_author/
   if( is_author() ) {
       // get_queried_object() returns current author in author's arvhives
       // https://developer.wordpress.org/reference/classes/wp_query/get_queried_object/
       $author = get_queried_object();

       // Generate meta description
       $description = sprintf( __( 'All posts by author %s', 'cyb-textdomain' ), $author->display_name );

       // Print description meta tag
       echo '<meta name="description" content="' . esc_attr( $description ) . '">';
   }

}