From the documentation:
Mongoose assigns each of your schemas an id virtual getter by default
which returns the documents _id field cast to a string, or in the case
of ObjectIds, its hexString.
So, basically, the id getter returns a string representation of the document's _id (which is added to all MongoDB documents by default and have a default type of ObjectId).
Regarding what's better for referencing, that depends entirely on the context (i.e., do you want an ObjectId or a string). For example, if comparing id's, the string is probably better, as ObjectId's won't pass an equality test unless they are the same instance (regardless of what value they represent).
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