Where does a sharded aggregation run?

Sharded clusters provide the opportunity to reduce the response times of aggregations because in many scenarios, they allow for the aggregation to be run concurrently. For example, there may be an unsharded collection containing billions of documents where it takes 60 seconds for an aggregation pipeline to process all this data. But within a sharded cluster of the same data, depending on the nature of the aggregation, it may be possible for the cluster to execute the aggregation's pipeline concurrently on each shard. In effect, on a four-shard cluster, the same aggregation's total data processing time may be closer to 15 seconds. Note that this won't always be the case because certain types of pipelines will demand combining substantial amounts of data from multiple shards for further processing (depending on your data and the complexity of the aggregation, it can take substantially longer than 60 seconds due to the significant network...