Starting a new company from a backup

Our company is currently using Desktop 2018 and we are in the process of upgrading to 2021.

We took over the books for a company and the audit firm that was previously managing their books sent me a thumb drive of their Quickbooks file.  They are running Quickbooks 2021.

First question:  Do I need to create a new company and then do restore backup using the thumb drive they gave me?  Will this upload all information including transactions?  I was going to follow the instructions provided here https://quickbooks.intuit.com/learn-support/en-us/manage-data/create-new-company-files-from-existing-ones-in-quickbooks/00/202624  but I would not want to remove the transactions from the copied file.

Second question:  Should I wait to do this until after we've completed the upgrade to Quickbooks 2021 or would it be ok to perform it in the older version?