This is a pretty good resource with a lot of details about Child Themes. I would suggest reading the entire thing.
You can override any file in your child theme by copying the parent theme’s file and placing it in the same directory of your child theme.
For example, to make edits to your footer.php, the file you would most likely need to edit to remove some of the 5 columns you mentioned, you would copy the parent theme’s footer.php and place it in the root of your child theme. Then you can freely make changes to this file.
I would suggest doing the above and playing around with the layout to try to achieve what you want. Then if you have more specific questions you can start a new question and get more specific answers.