The <head> and <body> tags are located in header.php and also header-blank-canvas.php, header-landing-page.php, and header-transparent.php.
These are the header templates that correspond to the various page templates. header.php is the default site-wide template used across the site, unless a page is set as one of the other page templates, in which case the relevant header file will be used. These are the only files containing <head> and <body> tags.
Rather than manually editing these header files, you can hook into the wp_head() function and the wp_body_open action in the functions.php file of your child theme, or use a plugin such as Google Tag Manager for WordPress that will do this for you.