I'm happy to see that you're interested in transitioning from QuickBooks Desktop to QuickBooks Online. Having the flexibility to work from anywhere using cloud technology will certainly benefit your business. I can shed some light on your options for converting from Desktop to Online.
If you've already converted your QuickBooks Desktop data to QuickBooks Online, that's great. If you need guidance on how to complete that process, see our Community article Migrating from QuickBooks Desktop to QuickBooks Online. You'll want to do this before importing any other items into QuickBooks Online, as the migration can only be performed while the company is still empty or it can cause issues to occur. Then the historical data will be moved and it'll only be the data entered into QuickBooks Desktop during the test period that you'll need to manually import to QuickBooks Online.
It sounds like you're looking to export customer and vendor transaction data into your QuickBooks Online after inputting it into QuickBooks Desktop. This can be done by exporting data from QuickBooks Desktop into an Excel or CSV file format, and then importing those files into QuickBooks Online. To export data from QuickBooks Desktop to CSV format:
For more information on exporting data from Desktop to CSV or Excel, see our Community articles Import/export CSV files and Import or export MS Excel files. Once you've completed those steps, you can then import the data into QuickBooks Online. The steps are:
Your invoices and other data from QuickBooks Desktop and QuickBooks Online will now be in sync. If this doesn't quite hit the mark, or if you need any further clarification please reply to this thread!