Populate id that references a nested Object

How do I populate an imbedded and nested object from another collection?

User Schema:

const walletSchema = new mongoose.Schema(
  {
    name: {
      type: String,
      required: true
    },
    balance: {
      type: Number,
      required: true
    }
  }
);

const userSchema = new mongoose.Schema(
  {
    username: {
    type: String,
    required: true
    },
    wallets: {
      type: [walletSchema],
      default: []
    }
  }
);

const User = mongoose.model(
  'User',
  userSchema
);
export default User;
`

Expenses Schema:

const expenseSchema = new mongoose.Schema(
{
userId: {
type: mongoose.Schema.Types.ObjectId,
ref: ‘User’,
required: true
},
walletId: { // ← will store the respective wallet id of the User
type: mongoose.Schema.Types.ObjectId,
ref: ‘User.walets.walletId’ // ← THE PROBLEM
},
amount: {
type: Number,
required: true
}
);

const Expense = mongoose.model(
‘Expense’,
expenseSchema
);
export default Expense;

I imbedded the wallets in the User object as I don't expect the user to have a thousand wallets and I will be accessing the wallets property frequently. Therfore I didn't separate it in it's own collection.

Is there a way to populate this field? or do i have to do it manually? The user's expenses will be shown in a table displaying each expense's information