Your question is about how to create a subcollection named account under a root collection named users.

You just need to do exactly the way you have shown in your question, using the set() method on the doc-account DocumentReference: as soon a first document will be added under the account subcollection, this subcollection will exist.

Same effect if you use the add() method on the account subcollection, as follows:

firestore().collection('users').doc(user.uid).collection("account").add({...});

And you even don't need to have a users collection, i.e. having a document in the users collection. This last point is a bit surprising at first sight, let's explain it with an example:

Take a doc1 document under the col1 collection

col1/doc1/

and another one subDoc1 under the subCol1 (sub-)collection

col1/doc1/subCol1/subDoc1

Actually, from a technical perspective, these two documents are not at all relating to each other. They just share a part of their path but nothing else. This is why the subDoc1 document (and its parent subCol1 collection) can exist without the doc1 existing. One notable side effect of this is that if you delete a document, its sub-collection(s) still exist.