Thank you for your reply.
Oh! the original cable. You're right. No, I don't use the original cable. Maybe I will borrow my wife's laptop: it has a USB C port. And it runs Windows instead of Debian, like my desktop. Or I will try to find the USB C to USB A adapter I have somewhere.
But, yes, I am in FastBoot mode and the cell phone was detected in the step before. I am using Brave browser and I think I have correctly disabled Brave Shields. And I have installed the android-sdk-platform-tools-common package on my Debian 12.