If you run the normal Ansible setup, there's one docker container running pictrs that's going to handle the image uploads. By default, it stores it into a volume that comes from the server's filesystem. So when you upload an image, it gets stored to the server's file system with the default settings.
Pictrs supports object storage, and you can get it to store the images to any S3 compatible storage.
Btw, if somebody here makes an instance with pictrs sending data to S3, it would be really cool if you would document it and write a guide how to enable it with a common Ansible deployment. It seems that the docker image doesn't take any environment variables, or at least there is no documented way how to turn on the object storage using the pictrs docker image. Maybe it's not implemented yet, and somebody needs to fork the docker image...