you will possibly need to edit content-single.php and author.php (i.e. copy these templates into your child theme and edit them there);
in content-single.php, find the section starting with this line:
<?php if ( get_the_author_meta( 'description' ) && is_multi_author() ) : // If a user has filled out their description and this is a multi-author blog, show a bio on their entries ?>
and ending with:
</div><!-- #entry-author-info -->
<?php endif; ?>
before the <?php endif; ?>
add a ‘else’ section where you copy all the html tags, but with default content;
example:
<?php else : ?>
whatever default bio you want to show
(use/replicate the existing html tags/structure for the default bio)
———-
similar in author.php; the section starts with:
<?php
// If a user has filled out their description, show a bio on their entries.
if ( get_the_author_meta( 'description' ) ) : ?>
and ends in:
</div><!-- #entry-author-info -->
<?php endif; ?>
again, add the ‘else’ code with the default bio before the ‘endif’ line.