The single post template file is used to render a single post. WordPress uses the following path:
1.single-{post-type}.php – First, WordPress looks for a template for the specific post type. For example, post type is product, WordPress would look for single-product.php.
2.single.php – WordPress then falls back to single.php.
3.singular.php – Then it falls back to singular.php.
4.index.php – Finally, as mentioned above, WordPress ultimately falls back to index.php.
https://developer.wordpress.org/themes/basics/template-hierarchy/#single-post
The template file used to render a static page (page post-type). Note that unlike other post-types, page is special to WordPress and uses the following patch:
1.custom template file – The page template assigned to the page. See get_page_templates().
2.page-{slug}.php – If the page slug is recent-news, WordPress will look to use page-recent-news.php.
3.page-{id}.php – If the page ID is 6, WordPress will look to use page-6.php.
4.page.php
5.singular.php
6.index.php
https://developer.wordpress.org/themes/basics/template-hierarchy/#page