How do I handle Many-To-One Relationships with Spring Boot + MongoDB

I'm learning how to use MongoDB with spring boot (Spring data MondoDB).

In my current code base that uses PostgreSQL, I have an API GET /api/orders/238392, which returns a model as follows:

{
    "orderId":238392,
    "status":"FULFILLED",
    "senNumber":"AB8893920",
    "customer":{
        "customerId":"lskj2409023471lskdjflsf",
        "name":"John Doe",
        "email":"john.doe@acme.com",
        "phoneNumber":"7326585306",
        "city":"Los Angeles"
    }
}

I now want to implement this user-case with MongoDB in the backend. This is my model

// Orders.java
@Data
@Document
public class Order {
    @Id
    private Long orderId;
    private String status;
    private String senNumber;
    private Customer customer;
}


//Customer.java
@Data
@Document
public class Customer {
    @Id
    private String customerId;
    private String name;
    private String email;
    private String phoneNumber;
    private String city;
}

My API now works fine. However, I have a problem with the relationship between Orders and Customers. In a relational world, I had the @ManyToOne on customer, as follows:

@ManyToOne
private Customer customer;

I cannot do this in Mongo (that's where I need help).

Now, to my question. I have two collections - Customer and Order. Both of them have CRUD APIs. How do I handle a ManyToOne relationship between Order and Customer.

If a user updates some properties of a Customer (name, email, etc), how does that change reflect in the Orders document?

I have the following options that come to my mind:

1. When I update a customer, I can update all the order documents with the new customer details (this doesn't seem correct - might cause a lot of performance overhead, i feel)
2. Using something like @DbRef - if that's the case, can I get some links to related documents or references.
3. Came across a library called RelMongo that allows you to use @ManyToOne annotations just like how you would in a relational world.

This could be a simple best-practices question, I'm not sure. However, any help goes a long way to improve my understanding!!

Thanks, Sriram