NSOperation and NSOperationQueue working thread vs main thread

My question is whether the database write operation occur in main
thread or in a background thread?

If you create an NSOperationQueue from scratch as in:

NSOperationQueue *myQueue = [[NSOperationQueue alloc] init];

It will be in a background thread:

Operation queues usually provide the threads used to run their
operations. In OS X v10.6 and later, operation queues use the
libdispatch library (also known as Grand Central Dispatch) to initiate
the execution of their operations. As a result, operations are always
executed on a separate thread, regardless of whether they are
designated as concurrent or non-concurrent operations

Unless you are using the mainQueue:

NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];

You can also see code like this:

NSOperationQueue *myQueue = [[NSOperationQueue alloc] init];
[myQueue addOperationWithBlock:^{

   // Background work

    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        // Main thread work (UI usually)
    }];
}];

And the GCD version:

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void)
             {
              // Background work            
             dispatch_async(dispatch_get_main_queue(), ^(void)
              {
                   // Main thread work (UI usually)                          
              });
});

NSOperationQueue gives finer control with what you want to do. You can create dependencies between the two operations (download and save to database). To pass the data between one block and the other, you can assume for example, that a NSData will be coming from the server so:

__block NSData *dataFromServer = nil;
NSBlockOperation *downloadOperation = [[NSBlockOperation alloc] init];
__weak NSBlockOperation *weakDownloadOperation = downloadOperation;

[weakDownloadOperation addExecutionBlock:^{
 // Download your stuff  
 // Finally put it on the right place: 
 dataFromServer = ....
 }];

NSBlockOperation *saveToDataBaseOperation = [[NSBlockOperation alloc] init];
__weak NSBlockOperation *weakSaveToDataBaseOperation = saveToDataBaseOperation;

 [weakSaveToDataBaseOperation addExecutionBlock:^{
 // Work with your NSData instance
 // Save your stuff
 }];

[saveToDataBaseOperation addDependency:downloadOperation];

[myQueue addOperation:saveToDataBaseOperation];
[myQueue addOperation:downloadOperation];

Edit: Why I am using __weak reference for the Operations, can be found here. But in a nutshell is to avoid retain cycles.