Can I enter historical data from another financial software into Quickbooks Online? How is this done?

If you are using QBO Plus, you may import the invoice data in bulk.

https://quickbooks.grsm.io/US

https://quickbooks.grsm.io/us-promo

But if you are using QBO SS or Essentials, or if you need to import multi transaction data type, you should utilize the importer tool.

https://transactionpro.grsm.io/qbo

https://partners.saasant.com/affiliate/idevaffiliate.php?id=5051_2

Hope it helps.