My understrap-child theme doesent have a head.php, footer.php

If you’re using a child theme you won’t necessarily have header.php and footer.php. If you don’t have them then the parent theme’s header.php and footer.php are used.

If you want to make a change to either of those templates you need to copy the file from the parent theme (presumably called understrap) into your child theme and make the modifications to the child theme version.