Find ObjectId of an element inside an array of docs, that is inside a model with Mongoose

Hello everybody. Let’s say I have this model:

const userSchema = new Schema({
    email: {
        type: String,
        required: true,
        unique: true
    },
    firstname: String,
    lastname: String,
    password: {
        type: String,
        required: true
    },
    activities: [{
        description: {
            type: String,
            required: true
        },
        status: String,
        from: Date,
        to: Date
    }]
}, { timestamps: true })

After I get the user with User.findById() , how can I get the ObjectId of an activity inside the activities array so I can perform (for example) delete and update operations? I can see every ObjectId with console.log() or in Atlas because MongoDB creates an ObjectId for every element inside the array even if I don’t specify the field _id for each element.