If you create a Template Override, it will be automatically applied globally; for example, you copied /components/com_content/articles/views/tmpl/article/default.php to
In this case, all articles would use the template override.
If you create a Layout Override, you will have to manually apply it to specific articles NOT LINKED TO THE MENU. If an article is linked to the menu, you need to apply an Alternative Menu Item Layout Override.
Use Layout Overrides to modify articles not linked to the menu; for example, category list and category blog articles are not linked directly to the menu.
For example, you copied /components/com_content/articles/views/tmpl/article/default.php to
Note the name change. By changing the name of default.php to newdefault2.php, you tell Joomla that this is a layout override instead of a template override.
To apply this example layout override to an article, open the article to which you want to apply the override, click Article Options, and from the Alternative Layout list, select the new layout. In this case, newdefault2.