How to add a fixed string to a mongoDB result like we do we in SQL Server?
You can use MongoDB's Aggregation Pipeline
to add calculated or constant fields to a query.

The $addFields
stage appends new fields to existing result documents if you don't want to specify
a list of all fields. However, since your example projects a subset of fields you
can also add new fields
using expressions in a
$project stage.

Your find() query would become a $match stage in aggregation. Here's an example showing a few different options (with comments) in the $project stage:

db.product.aggregate([

    // The initial $match stage can take advantage of available indexes
    { $match: {
        "company.cad": {
            $in: [" ", "14940"]
        },
        "tradeItemMeasurements.netContent.measurementUnitCode": {
            $ne: null
        },
    }},

    // Project the desired result fields
    { $project: {
        // Fields that already exist
        "_id": 1,
        "brandNameInformation.brandName": 1,
        "childTradeItems.gtin": 1,
        "company.cad": 1,
        "placeOfProductActivity.countryOfOrigin.countryCode": 1,
        "referencedFileInformations.uniformResourceIdentifier": 1,
        "tradeItemClassification.gpcCategoryCode": 1,

        // constant values
        "ACTIVE": true,
        "language.code": "PT-BR",

        // calculated values (in this case, size of an array)
        "itemCount": { $size: "$tradeItems" },

        // literal values (that might otherwise be interpreted as expressions)
        "batch.batchNumber": { $literal: 1 },
    }}

])