Aggregate Mongo queries accept multiple steps, including SQL-style joins using $lookup. This will add a new property to the results, with an array of documents from the other collection. You can then use $unwind to make each result map to a single document from the other collection. Finally, $project can help you select only the fields you care about.

For example, assuming collectionA has fields id, valueA, and otherId; and collectionB, id and valueB, you can do this to join the two as if they were SQL tables, selecting both ids and both value* fields: