Template files used in WordPress themes are determined by their priority.
For example, the file single.php is used to display the post page, but if this file does not exist, the file index.php is used.
In other words, single.php, has higher priority than index.php, so single.php is used.
In addition, the top page file has priority, such as front-page.php > home.php > index.php.
(">" Indicates priority.)
In this case, front-page.php has a higher priority than home.php, and home.php is higher than index.php.
Below, each priority is listed in chart form.
A. is the highest priority.
Continue with B. , C.～