As of the moment, there isn't an integrated way to import your inventory items and customers' balances. But you can import them as a list a manually enter the respective balances once done.
That being said, here's an article you can read to learn more about how you can import your list: Move your lists to QuickBooks Online.
Additionally, I've also included this helpful article for the steps in creating a CSV file of your bank transactions for manual upload: Format CSV files in Excel to get bank transactions into QuickBooks.
If there's anything else that I can help you with, please let me know in the comments. Stay safe!