Ok that’s two questions:
InsertMany vs BulkWrite
Using BulkWrite you can do many operations in a single connection to mongoDB. Internally, InsertMany uses BulkWrite, so there’s no difference, it’s just for convenience.
This question was already solved.
Sync vs Async
When you perform a sync operation, your aplication will wait for MongoDB to finalize the work. With a async operation you can perform many operations at the same time. Server, and client side.
This was already solved too.