Change Default Layout for Just Blog Posts

What needs to be done is theme dependent. You may just need to remove the code that loads the sidebar from a particular template. Or create another template that does not have the sidebar code to begin with. Depending on how the template is named, it will only be used in certain situations. See https://developer.wordpress.org/themes/basics/template-hierarchy/

You could also manage the loading of the sidebar template with a conditional tag.