How to boot Fedora when it is installed on the same disk with Debian

Hello to everyone.

I’ve installed Debian (11) and Fedora (37) on the same disk (and I plan to install even Arch) and I would like to boot one of these OSes depending on my needs. The first os that I’ve installed has been Debian (I have used the automatic partition scheme) and then Fedora (even here I have left the Fedora installer to chose how to partition the space left free by Debian). At the moment I haven’t installed yet Arch,because I’m able to boot Debian,but not Fedora and I would like to understand what’s wrong before to do it. Obviously Debian before and Fedora after,taken all the space available on the disk,so I have used gparted to resize their root partitions. While I tried to understand what could be broken,I’ve took some screenshots to show you how the disk is partitioned. The problem is that the Fedora entry does not show on the grub menu even after writing “os probe” from Debian (it’s true using qemu-kvm and even on the bare metal). And it does not boot even using the EFI partition that it creates and that’s different than the EFI partition created by Debian and modified by Fedora on the first partition of the disk. Anyway,I understand that’s hard for you to understand what’s happening without to give a look at the pictures that I’ve got,so here you can :

As you can see below,the fedora installation is not detected :

marietto@marietto:~$ sudo update-grub

[sudo] password di marietto: 
Generating grub configuration file ...
Found background image: /usr/share/images/desktop-base/desktop-grub.png
Found linux image: /boot/vmlinuz-5.19.0-15.2-liquorix-amd64
Found linux image: /boot/vmlinuz-5.10.0-21-amd64
Found linux image: /boot/vmlinuz-5.10.0-18-amd64
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
Found Windows Boot Manager on /dev/nvme0n1p1@/efi/microsoft/boot/bootmgfw.efi
Found Ubuntu 22.10 (22.10) on /dev/nvme0n1p4
Found Ubuntu 22.04.2 LTS (22.04) on /dev/sdb1
Adding boot menu entry for UEFI Firmware Settings ...

I hope you can help me to understand how to accomplish this task. Very thanks.

In your case I just would select one as my favorite and work with this. The rest I would solve with “Virtual machine manager”.

This way you can even use two or three OS’s together without the need to dual-boot.

If your system is UEFI capable, you can have one EFI partition per distribution.

In this way, we can always use the UEFI boot preference / boot menu to choose which one to boot.

Can you enter from Bios the UEFI boot manager. Sometimes it’s F9 or another key to enter a boot menu. Good chance that you find debian and Fedora there. If you select Fedora, you probably get into Fedora’s grub and find Debian there too. Then set Fedora as default in the UEFI manager or Fedora’s efibootmgr.