Should just need to be logged in with their Autodesk ID. If you weren't a licensed user of 2019 when 2019 was out, I don't think a license of 2021 will let use use it. Prior version access (as far as I know) only works if you were a subscriber or on a maintenance subscription when 2019 was released.
There's not really a "File" to use. If an Autodesk ID doesn't work, you an try activating using the serial number which can be accessed via the Autodesk Accounts portal by your contract manager or secondary admin.
Darren J. Young
http://www.darrenjyoung.com/
https://www.linkedin.com/in/darrenjyoung/