Where to place PHP Code?

Hi community!

I would like to add a Privacy Policy checkbox to the default WP registration Form. I have found this here: https://wpbrigade.com/add-privacy-policy-checkbox-registration-form/

Now I am just wondering – where do I paste this code? Does all of that go into a functions.php file (in a child theme I guess?) or is it saved as a plugin?

Thank you!