Object storage can be used either as primary storage or as external storage. When it is used as primary storage, the object store only store the file content by using a unique identifier. No metadata are stored in the object store: no files names, no directory structures, etc. Nextcloud assumes exclusive access over the bucket being used.
This has benefits and downsides: performance is better when you use an object store as primary storage, but it restricts being able to access the files from outside of Nextcloud.
(From: Object Store as Primary Storage)
hth.
Much and good luck,
ernolf