Thank you for your reply Antonio.
I want to downgrade because my internet connection doesn’t allow me to depend on the cloud and my Adobe collection (along with other applications) no longer works with Big Sur. So, I did not buy a "new" computer but a 2019 MacBook Pro reconditioned by Apple.
I first connected the two computers together with USB C and tried with migration to transfer the old into the "new", but the "new" computer refused.
Then I tried with Recovery and my last TimeMachine backup, but the message was: "You can't restore this backup because it was created by a different model of mac”.
So I used Carbon Copy Cloner to make another backup, but I didn’t know how to use it because Recovery window seems to recognize only TimeMachine backups.
Now I have little possibility on the screen, hard drives "Macintosh HD" and Macintosh HD - Data" having been erased); I did not even dare to click "restart" for fear of being blocked.
Let’s add that, before erasing the "new" computer, I made a TimeMachine backup, but then I realized that only the disk "Macintosh HD - Data" had been saved. So (given my weak internet connection), any going back (Big Sur) seems to me compromised.
I followed the links you gave me. Thanks.
If there is nothing simpler to do (is it not possible to use CCC Back up?) I will try to restore with Apple Configurator 2.