If I create a new Desktop QBs file to enter 2020 transactions, is it possible to transfer all past transactions up to 2019 from an older file into this new file later on?

I would recommend you to keep the data in separate files. You may utilize the consolidating feature available in Enterprise edition. But should you prefer to import the past data into the new file, you may use the IIF feature. If you are a newbie to IIF, consier using a 3rd party importer tool.

https://transactionpro.grsm.io/qbd

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