Adding HTML code under header on all post? (like an Amazon affiliate disclosure)

Looking for something like this:
https://imgur.com/a/XEwDX3r

Looking to add a piece of code saying the Amazon affiliate disclosure thing "I may earn an affiliate when clicking my links". I heard I had to edit the child theme's single.php (or something like that), but I really don't know how to do that. I'm also very new to PHP.

Also for security purposes, I don't want to use any plugin

Thanks!