If you have a child theme, WP attempts to find the template file in the child theme folder first, then tries the parent theme folder.
For the footer, when calling rendering the footer using get_footer($name), WP will search for
child-theme/footer-{$name}.php
child-theme/footer.php
parent-theme/footer-{$name}.php
parent-theme/footer.php
in that order and execute the first one it finds. If you have a footer.php in your child theme folder and in the parent theme folder, it will execute the child footer.php and simply ignore the parent theme footer file.