One-to-One optional relationship with both ends optional and both FKs

In any one-to-one association, EF uses only one foreign key. When the association is required, the foreign key will also be the primary key of the dependent entity, as explained here.

When the association is optional, both entities should be able to exist independent of one another. So their primary keys can't be foreign keys, because PKs can't be optional. Here an additional nullable FK field is required to establish the optional association.

In your case, technically it doesn't really matter which entity has the FK field (logically, it may). I've used this model:

public class Review
{
    [Key]
    public int ReviewId { get; set; }
    public virtual Payment Payment { get; set; }
}

public class Payment
{
    [Key]
    public int PaymentId { get; set; }

    public Review Review { get; set; }
}

With this mapping:

public class ReviewConfiguration : EntityTypeConfiguration<Review>
{
    public ReviewConfiguration()
    {
        // One-to-One Optional
        HasOptional(s => s.Payment)
            .WithOptionalDependent(s => s.Review)
            .Map(s => s.MapKey("PaymentId"));
    }
}

(So, apart from Payment.ReviewId, this is identical to the model + mapping in your question).

Now I can do things like ...

db.Set<Review>().Add(new Review { Payment = new Payment() });
db.Set<Payment>().Add(new Payment { Review = new Review() });
db.SaveChanges();

... where db of course is a context. The content of both tables now is:

PaymentId
-----------
1
2

ReviewId    PaymentId
----------- -----------
1           1
2           2

And I can query the data bidirectionally like so:

var data = db.Set<Review>().Include(r => r.Payment).ToList();

or

var data = db.Set<Payment>().Include(r => r.Review).ToList();

But instead of ReviewConfiguration, I can also use...

public class PaymentConfiguration : EntityTypeConfiguration<Payment>
{
    public PaymentConfiguration()
    {
        // One-to-One Optional
        HasOptional(s => s.Review)
            .WithOptionalDependent(s => s.Payment)
            .Map(s => s.MapKey("ReviewId"));
    }
}

Now there will be an FK field ReviewId in table Payment, and the rest of the code works without changes.