Hi!
To backup files you can do it as you normally do, for more privacy use a physical device like a USB or external hard drive, also you can use any cloud service you want.

To backup Qubes you can read this article:
https://www.qubes-os.org/doc/how-to-back-up-restore-and-migrate/

I’ll suggest to read the Qubes documentation, it’s very extensive and well explained, they have answers for lot of questions as well in their website.

Greetings!
Diego