Having never used boto3, I found this is the docs, so it looks like they make it pretty simple:
https://boto3.amazonaws.com/v1/documentation/api/latest/guide/s3-example-download-file.html
Since it accepts a file handler, I don’t see why you couldn’t shove it into a file stream and operate it in memory without even writing it to disk.
(Lets say you are going to immediately load it into a pandas dataframe or something, why write to disk)