child theme

Thank you. I had already copied all the code in the loop-index.php file, changing only the line that interested me and put it in the child theme. If I do this, however, there are 2 problems:
1) The loop-index file is not the only one I have to modify. There is also the file helper-social-media.php and others;
2) If I copy all these files in full and an update of the parent theme arrives, the copy of these files will be obsolete.

For this I would like to insert in the child theme only the line that I am interested in modifying, not all the code. But I tried, for example, to insert only this code in the child theme, in the loop-index file:
<?php
$ blog_content =! Empty ($ avia_config [‘blog_content’])? $ avia_config [‘blog_content’]: “excerpt_read_more”;
if (is_single ()) $ blog_content = “content”;

but an error is generated. How can I do?

All pages are empty, without text and images. I would like to know how and where to write only the change that interests me, without copying entire php files