Modifying Registration Fields:
BuddyPress provide documentation on how to modify the registration fields here: https://codex.buddypress.org/getting-started/guides/modifying-the-registration-form/
Modifying Registration Templates:
If you're using BuddyPress, then the registration page is being generated by BuddyPress and not the theme. You can add the following directory within your child theme folder /buddypress/members/register.php to override the default registration template.