Aggregation in mongodb is refered as the process of performing different type of operation on multiple documents with in a collection. This is done by using pipeline, which is made up of different stages. A stage output is input for another stage.

Different type of Aggregation pipeline:

$match : It is a operation which filters documents based on specified criteria.

example:

const Channel = await User.aggregate([
{
    $match:{
        username: username?.toLowerCase()
    }
},

$lookup : performs a left outer join to a collection in the same database. The $lookup stage adds a new array field to each input document. The new array field contains the matching documents from the "joined" collections. The $lookup passes these reshaped stage to next stage.

{
   $lookup:
     {
       from: "subscriptions", //select from the collection
       localField: "_id", // localfield of the collection
       foreignField: "channel", // foreignfield of the collection
       as: "subscribers" // Name of the returned collection
     }
},

$addFields: $addFields add new fields to the output output document that contains all the existing field from the input document

{
   $addFields:{
    subscribersCount:{
        $size : "$subscribers"
        },
    channelSubscribedToCount:{
        $size:"$subscribedTo"
        },
    isSubscribedTo:{
        $cond:{
                if:{$in:[req.user?._id,"subscribers.subscriber"]}
                then:true,
                else:false
            }    
        }
    }
}

In this example, $cond is adds field with some computation and condition.

$project: it passes the document with the requested fields to the next stage in the pipeline. The field can be existing fields from the input document or newly computed fields

{
    $project:{
        fullName:1,
        usename:1,
        email:1,
        subscriberCount:1
    }
}

In $project 1 represents that it should pass these fields which have 1.

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A big big thank you Hitesh Choudhary sir , who made a video on aggregation pipeline in hindi.