S3 is really simple.
Honestly idk why you even bought this course...
File A goes in bucket B at key C. That's it. You only need 2 things to access an object in S3:

Buckets have globally unique names because they are part of the URL used to access them. If I make a bucket called "bd-vids", you can't make a bucket called "bd-vids", even if you're in a separate AWS account. This makes it really easy to think about where your data lives.
Try to make a bucket called bootdev. It won't work. Ha! I took that name first. A lot of organizations use a company specific prefix to ensure their bucket names are unique. For example, bootdev-user-images.