Have a new website and want to install Google Tag Manager (GTM). The GTM code requires a (second) tag. According to this site (https://www.wpexplorer.com/google-tag-manager-wordpress/) you can copy files from the main theme to the child theme. "Google Tag Manager installation codes into the header.php file."
However, my concern is that it will break the site. We first made child theme (with only styles.css and functions.php) and then designed the website.
Or is there another way to install the second GTM tag to the site?