Currently, it is only possible to index one collection. This means the Firestore extension will not index a collection and its sub-collection by default.
However, the Algolia Firestore extension can be installed multiple times, monitoring different collections and pushing the changes to a central Algolia Search Index or a separate Index for each collection. Therefore, it is possible to index sub-collection only with the workaround of installing the extension multiple times.
You can read more about this in this github issue.