WordPress Template Hierarchy:
You should read about WordPress template hierarchy.
This question also has some related answers.
In your case, the most likely scenario is:
The explainer had set the latest posts as the home page (this is the default option) and you have set a static front page as home page. Since his is blog archive, it's being loaded by
index.php and yours is a page, so that's being loaded by
This is just a typical scenario, but there may be other settings that are affecting which template file is being loaded.
Changing the default setting:
If the above is your scenario, then changing the following setting from WordPress
wp-admin -> Settings -> Reading -> Your latest posts (then
Save Changes) will make it the same for you and the explainer. Check the screenshot below: