Having a Child Theme is a good start; if you were to make changes to the main theme, those changes would get overwritten on a theme update.
In order to solve your problem, you will need to change how the various templates work. And in order to determine which template to use, you need to understand the WordPress Template Hierarchy, explained here in the Codex https://developer.wordpress.org/themes/basics/template-hierarchy/ .
Based on that information, and the files in your theme, you would need to copy the appropriate template file from the theme folder and place it in your Child Theme folder. Then edit that file (in the Child Theme folder) to make the changes on how different elements of the page are ‘built’ by WP using the template.
Specifics depend on the actual theme code (which varies from theme to theme). But, with a bit of PHP knowledge, you should be able to figure out what pieces of the code to change to not display the featured image wherever the theme puts it.
Because your new template is in your Child Theme folder, that template is used instead of the one in your parent theme. So changes you make will be used.