I just wanted to get this confirmed though I strongly feel that snapshot policy takes a backup only of an index.
I have configured ILM to delete indices older than a week. Snapshot policy will be taking backups of the same indices everyday and storing them in S3 buckets.
What happens when week old indices are deleted from the cluster? My understanding is that the snapshot policy will not delete the snapshots of indices which were deleted. It will be driven by its own retention policy which I will set.