Will macOS 11 Big Sur Recognize External Hard Drives That Were Formatted Using macOS 10.15.x Catalina

A friend gave me a new iMac for Christmas 2019. He had gotten it from one of those financing places. My friend died last week, and I have been told I have to give the computer back to the company.

I will be able to afford to buy another iMac next Spring, but by then it will be macOS 11 Big Sur, instead of macOS 10.15.x Catalina.

I have a back up external drive and a couple of other externals for music, documents and miscellaneous files. Is it possible that the new OS will be able to read the external drives I have in use now, since they are formatted using Apple's new filing system, which I would think the next OS will use as well?

I do not want to lose anything, thus the questions, but am afraid of what might happen once I plug them into a computer with the new macOS 11 Big Sur. Is it possible that instead of using my back up, once I get ready to box up this iMac, that I would be able to use Migration Assistant instead? But if I do not use my back up, do I stand a chance of losing access to some apps which I have purchased?

Any help, ideas, feedback or resources would be greatly appreciated.

This really is something, my friend's death and having to return the computer, I had never expected to have happened.

Thank you.