Multiple Post Layout in a theme

I want to have different post layout in a theme and that’s WITHOUT a plugin. The post template section will show in metabox in dashboard(whenever someone wants to add new post). Unwilling to use custom post type plugin,therefore kindly don’t post them. How do I achieve this ? should I create new different single.php file version like – single-layout1.php/single-layout2.php files and do some conditional check or is there any better way ?

Couple of Google search results –
1. https://wordpresshero.com/tricks/different-single-post-pages-in-wordpress.html/
2. https://www.elliotjaystocks.com/blog/tutorial-multiple-singlephp-templates-in-wordpress/
3. https://wordpress.stackexchange.com/questions/35415/wordpress-single-php-different-layouts-projects-posts