Volume vs Object Storage
Volumes behave much like a physical drive and contain an ext4 file system, just like a normal drive. Any application that can read and write from a normal file system can use block volumes. Files are organised hierarchically in directories.
Object storage is not a file system, it treats file as individual objects that are stored in containers (buckets.) It is a flat file system so all files are stored in a container at the same level. You can “virtually” organise files into folders by including a full “path” as the file name (e.g. /path/to/file/filename.txt) but they are still physically stored in a single hierarchy.
You use an API to read and write the files, so applications need to integrate with object storage’s API to use it. If the files ACL is set to public, object storage can serve the files over HTTP/S.