If so, how does it iterate in key order?

I should clarify - the problem with Firestore is the index tablets. The indexes inherently need to be inorder or you can't perform ordered queries.

It can't. You can only perform range queries and the like on "sort keys." All sort keys are on the same partition, so has limits on storage size and throughput.

Partitioning is very explicit in DynamoDB, for better or for worse. Harder to shoot yourself in the foot, but also limits what you can do.