Thank you! This is the first explanation that I think fully explains what I was confused about. So essentially the prefix is just the first N bytes of the object's name, where N is a per-bucket number that S3 automatically decides and adjusts for you. And it has nothing to do with delimiters.

I find the S3 documentation and API to be really confusing about this. For example, when listing objects, you get to specify a "prefix". But this seems to be not directly related to the automatically-determined prefix length based on your access patterns. And [1] says things like "There are no limits to the number of prefixes in a bucket.", which makes no sense to me given that the prefix length is something that S3 decides under the hood for you. Like, how do you even know how many prefixes your bucket has?

[1] https://docs.aws.amazon.com/AmazonS3/latest/userguide/optimi...