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To transfer documents from one collection to another in MongoDB, one can use the "aggregate" method and specify the output collection. Here are the steps:

1. Connect to the MongoDB server and select the database that contains the source collection and the one that will receive the transferred documents.

2. Use the "aggregate" method and specify the source collection as the input.

3. Use the "out" option to specify the output collection where the transferred documents will be stored.

4. If necessary, use the "match" and "project" stages to filter and transform the documents.

5. Execute the aggregation pipeline.

Here is an example code snippet that will transfer all documents from the "sourceCollection" to the "targetCollection" in the same database:

db.sourceCollection.aggregate([
  {
    $out: "targetCollection"
  }
])

Note that the "out" option will overwrite any existing documents in the target collection with the same "_id" field. If you want to preserve the existing documents, you can use a temporary collection as the output and then merge the collections manually.