mongodb distinct operation on nested object inside array

With the current data structure there's no way avoiding using $objectToArray on all the data and then grouping. which will make this pipeline very inefficient.

db.collection.aggregate([
    {
        "$addFields": {
            "newField": {
                "$objectToArray": "$$ROOT"
            }
        }
    },
    {
        "$unwind": "$newField"
    },
    {
        "$unwind": "$newField.v"
    },
    {
        "$group": {
            "_id": "$newField.v.browser"
        }
    },
    {
        "$match": {
            // this null is due to the _id field, you could also filter "newField" before unwinding instead.
            "_id": {"$ne": null}
        }
    }
]);

With that said I recommend you consider re-structuring your data, With some like:

{
    "_id": ObjectId("5e049ebc8e935c407f68c190"),
    date: "2020-04-26",
    data: [
        {
            "browser": "Chrome",
            "ts": "2020-04-26 10:45"
        },
        {
            "browser": "Edge",
            "ts": "2020-04-26 10:45"
        },
        {
            "browser": "Firefox",
            "ts": "2020-04-26 10:46"
        }
    ]
}

Now you can easily achieve the same result (and many others) like so :

db.collection.distinct("data.browser")