We've introduced a new groupObjects argument to bulkOperationRunQuery and bulkOperationRunMutation mutations in the GraphQL Admin API that allow clients optionally to disable grouping and benefit from faster and more reliable bulk operation job runs.
Great question. I checked here and it is reasonable to assume that they will be somewhere below the parent object and what is most likely to be what happens, however do to the async nature of the bulk queries with grouping disabled, we can’t guarantee that it will always happen that way.