Copy the AdSense code and between the <head> and </head> tags

Hello, I would like Google to automatically place ads on my site. I have to add a code between and .

<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-00000000000000"crossorigin="anonymous"></script>

In which file do I need to add the codes?

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