The “system” of template files is not too complex.

— single.php vs single-post.php

It’s good sense to build as simply as possible in your template structure and not make more templates unless you have real need for them. Therefore, most theme developers don’t create a single-post.php file because single.php is specific enough. For the most part, all themes should have a single.php.

— content.php vs content-single.php .. I read the top answer in https://wordpress.stackexchange.com/questions/239401/are-content-php-and-content-single-php-the-same

Most of the time in my case, the page I hit has “single” format, so content-single.php is used, instead of content.php. However, consider:

get_post_format() returns (for example) video. But you don’t have content-video.php template part file. The engine would “fall back” on content.php