Thank you for writing in!

Add this code at your child theme’s functions.php

add_filter( 'the_content', function( $content ) { return $content.( is_singular() ? '[author]' : '' ); } );

Cheers!

I just found this topic and followed your advice. The thing is, now the authors name and avatar appear in every page of my site and not just in the blog posts as I wanted. Is there anything I cad add to the code you provide to make it work just for the blog posts? Is there any way to turn it off in the rest of the site?

To do so, add the following:
add_filter( 'the_content', function( $content ) { return $content.( is_single() ? '[author]' : '' ); } );