Nope. Didn’t work. 
The content of my grub.cfg:
GRUB_DEFAULT=0
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR=lsb_release -i -s2> /dev/null || echo Debian
GRUB_CMDLINE_LINUX_DEFAULT=“quiet splash iommu=soft”
GRUB_CMDLINE_LINUX=" "
GRUB_DISABLE_OS_PROBER=false
‘GRUB_DISABLE_OS_PROBER=false’ - this line was already uncommented by default. After editing the grub.cfg, I rebooted the PC and nothing changed. The same Debian boot menu showing only Debian. 
Did you run update-grub?
Yes. Obviously. After I ran the command
sudo update-grub
I rebooted the PC and when booting, it showed ‘Welcome to GRUB’ once (like it was shown at time of booting into Fedora), just a flash, and then it showed the Debian menu and booted into Debian.
From Debian, paste here the output of these commands please!
sudo parted -l
sudo blkid
mount
cat /proc/cmdline
You need to set the GRUB_TIMEOUT=5 a little higher, say to =20!!!
Why? 5 seconds should be enough to choose a system to boot, if not the default…
For some reason OS prober did not find Fedora.
Either different install type (legacy versus UEFI) or Fedora was mistakenly overwritten upon installing Debian.
Or I don’t know, we’ll see what those commands output.
Sorry brother, it was impossible for me to post the output of those commands because Dabian recognised only the bluetooth part of my bluetooth+wifi dongle. It did not recognise the WiFi. So, I couldn’t connect to the internet. So, I booted into Windows and wiped the whole drive. Tomorrow I will again try to install both OS with a dual boot. This time I will go for the Debian installer from its boot menu without booting into the live OS. Let’s see what happens. Goodnight.
Maybe for some but my 76 year of age has not helped with my reflexes or eyesight!!!
One time I had 4 Linux Distro’s on one 500 GB HDD. Not in production, just for testing.
I will not go into details because 1) it was a couple years ago and 2) Neville already did a great job explaining the process.
Isn’t Fedora BTRFS? That is what I just dealt with in getting MX Linux (a Debian-based OS) to show Garuda (an Arch distro BTRFS) on it’s grub menu. Just a hunch, but may have to deal with this in a different way as btrfs OS usually like to control the other Linux OS and regular grub does not see a btrfs install as it uses @subvolumes.
Sheila
That is not the grub.cfg file.
That is /etc/default/grub
Of course not. After editing /etc/default/grub, you have to do update-grub for the edited changes to be incorporated into /boot/grub/grub.cfg.
Then you can reboot and it will work.
You really have to be careful with your terminology. Dealing with grub needs close attention to detail.
Oh dear, I hope not.
That changes everything.
@Skywalker71 can you check what filesystem Fedora has please. . … I think what I told you will only work if it is not Btrfs.
I thought so. https://fedoraproject.org/wiki/Btrfs
Sheila
Hi Sheila,
Thanks for that.
Can you refresh my memory? I seem to recall btrfs systems need a separate /boot partition that is ext4?
That does not mean they need go be as complicated as Garuda.
I am trying to think through whether that would affect @Skywalker71 trying to dual boot Fedora with Devuan?
When I had Fedora 39 & 40, it was a single boot. But with Garuda, it only required a separate EFI of 512 MB at least. Installing it on its own partition with MX the first OS installed, it went through the setup of creating its own EFI/Boot and then it became the controlling grub OS. That was fine as MX Linux showed up in that grub menu. But the reverse is not true.
I still keep finding others thinking chainloading is easier than the modifications to grub, etc. And others prefer to just put Fedora 1st in BIOS (UEFI). But I would not want to have to get into BIOS to switch between the OSs.
But here is a relevant link: where although the answer is at the top, I suggest reading below through the conversation thread about the EFI partitions and what I had ran across before - something about “shim”
Maybe this will help.
Sheila
I just took a look via Disk Manager in MX Tools at my Garuda EFI (more about that sudden popup in my other thread) and it is not ext4 it is vfat just like the MX EFI.
Sheila
One other thread found on https://wiki.debian.org/Btrfs%20migration is about converting an ext4 to btrfs, but on down the page it gives instructions on how to get it found in grub. It looks like it takes a bit of effort, but when reading my solutions for Garuda, I thought it sounded complex as well. But after doing the modifications, I ended up thinking it wasn’t as hard as it sounded.
Sheila
Hang on, EFI is /boot/efi not /boot.
They are different things.
Remember this topic?
Now you know, how much noob I am.
So, if you want to teach me how to dual boot between Fedora and Debian with Fedora being the primary, you have to specify step by step like, "1st you do this. 2nd, you do that… " like this… just like spoonfeeding a baby. 
(I actually do that when I teach someone video editing or photography…lol… It actually refreshes memory 
)
I did just that. You and @kovacslt clearly mentioned that and I also have some idea that after updating grub.cfg, without rebooting, it would not come into effect and I also mentioned it in the 22nd post here. 
Unfortunately the file system of Fedora is indeed btrfs starting from Fedora 33.
Now I demand a spoonfeeding… regarding this Fedora Debian dual booting obviously.
FYI, I have installed Fedora already, from scratch and have updated. I will not install anything till I learn how to dual boot these two OS. 
My work laptop has btrfs filesystem with Gentoo which controls the grub. Second os is Debian. I can check my setup when I’m at work. Can’t remember what filesystem Debian partition has but I assume ext4. I used Debian iso to install it alongside Gentoo and did manual partitioning during install.
I’ll get back to this later today.