I would like to integrate Debian,Fedora,Arch or maybe some other distro into one single,homogeneous and flexible operating (OS) context.

 
I would like to use Debian 11 as the host OS and the rest of the OSes (Fedora,Arch) will be virtualized within different virtual machines
(I don't want to use containers,I know that they are a better choice,but I want to understand if a virtual machine is good anyway when it is very well configured). The idea is also to use qemu + kvm with the nographic and no vga options enabled,without virt-manager or boxes.

I don't want to open and close the virtual machine graphical interface every time I want to run an application. I find it much more comfortable if I can run every application and command within one only terminal by premising,for example,a letter indicating to which distribution the command or application belongs to. Or I could tell xpra to open the fedora or the arch linux terminal. For sure it will be much faster to configure,but it has less scenic effect.

I don't take in consideration the apps that can be installed through flatpaks,snap or app images,because I don't like to use these systems.

 
I'm a traditional user of linux,I have used it since the 90's. I prefer to use the traditional packaging systems used by debian,fedora,arch.
I've read that in some cases the snap system in Ubuntu contains obscure parts of code. And I've checked by myself how much can those systems complicate the management of the OS if they get jammed or if one does not want them because he/she/ wants to streamline the system.

So,I really like the idea to use apt/apt-get, dnf/yum, pacman and yay all together from one only shell, just like you'd use them on their native distributions. I may want to install or remove an application on one of those OSes because it does not work anymore and you have no time to fix it.

You know that sometimes it takes time to fix the broken part,time that you don't have. Luckily it still works for the remaining one or two OSes. I may like to use some specific distro just because I like,without particular reason,for some time. Maybe there are also different technical reasons why I want to have that fusion,tied to the libraries used. Some libraries could improve the stability and the speed of the systems,some others could do it less. It seems that some good distros are coming out these days,following the same idea : a couple of them are : blendOS,BedRock.

Now,I'm on Debian 11 (where I'm using the default qemu version 5.2) and I'm trying to boot Fedora 37 using the following qemu commands :

/usr/bin/./qemu-system-x86_64
-enable-kvm
-m 8G
-machine q35,accel=kvm
-smp 8,cores=4
-cpu host,kvm=off
-smp 2,sockets=1,cores=2,threads=1
-drive if=pflash,format=raw,readonly=on,file=/usr/share/OVMF/OVMF_CODE_4M.fd
-device ich9-intel-hda
-device hda-output
-usb -device usb-kbd
-device usb-mouse
-device virtio-net-pci,netdev=net0
-netdev tap,id=net0,ifname=tap0,script=no,downscript=no,vhost=on
-device ich9-ahci,id=sata
-drive id=SystemDisk,if=none,snapshot=off,cache=none,media=disk,format=raw,file=/dev/sdd
-device ide-hd,bus=sata.4,drive=SystemDisk
-device bochs-display
-device pcie-root-port,bus=pcie.0,multifunction=on,port=1,chassis=1,id=port.1 \

At this point your script comes into play. The idea is to use your update-linux script to update automatically the Fedora VM that's running under the hood. This seems to be the missing piece for my project. But it's not the only missing piece. My idea is to keep synchronized all the VMs that are running silently,so the project still needs two pieces of code : your script should do the same for the arch and the Ubuntu VMs. The idea behind is to have a sort of immediate backup if one of the VM gets corrupted. But this backup is represented by the remaining and still working VMs that's still running in memory. 

Do you like the idea ? I'm not a coder,so I'm not able to replicate your script to make it work for Ubuntu and arch.  You can,if you like the project and you want to collaborate with me. Let me know,thanks.