Where are my BTRFS subvolumes?

You can move your / to a subvolume this way:

1. Create a shapshot of your filesystem.

    btrfs sub snap / /@
   

2. Mount the new subvolume to /mnt.

    mount -o subvol=@ /dev/vda2 /mnt
   

3. Chroot to the subvolume and update grub.

    cd /mnt
    mount -o bind /dev  dev
    mount -o bind /sys  sys
    mount -o bind /proc proc
    chroot /mnt
    update-grub
    exit
   

4. Update /mnt/etc/fstab adding there subvol=@ as an option.

5. Reboot. You will boot to the subvolume. Make sure that it is the case by

    mount | grep vda2
   

It should show something like

   /dev/vda2 on / type btrfs (rw,relatime,space_cache,subvolid=257,subvol=/@) 

6. Now you can mount the top subvolume somewhere and delete its contents except /@.

Here is a solution which works on Ubuntu Server 20.04. It creates @ subvolume before the first boot and removes all files from / volume.

1. Do the installation of Ubuntu 20.04 with BTRFS root partition, but don't reboot after the system installation.

2. Switch to terminal (Alt+F2).

3. Switch to root user and umount all devices other than the BTRFS partition:

   sudo -i
   umount /target/boot/efi
   umount -l /target/run
   umount /target/cdrom
   

   If you have other mount points (i.e. /home) umount them too.

4. Create @ subvolume and move all files into it:

   cd /target
   btrfs subvolume create @
   ls | grep -v @ | xargs mv -t @
   

5. umount your BTRFS partition and mount it again pointing this time to the @ subvolume. It is also a good time to define some extra mounting options (in my example there are some recommended options for SSD devices). I assume BTRFS volume is under /dev/sda2 (adjust accrdingly)

   cd /
   umount /target
   mount -o subvol=@,ssd,noatime,space_cache,commit=120,compress=zstd:2 /dev/sda2 /target 
   

6. Now it's time to complete the system by mounting all necessary devices and then switch to it with chroot (I assume your boot partition is /dev/sda1)

   mount /dev/sda1 /target/boot/efi
   mount --bind /proc /target/proc
   mount --bind /dev /target/dev
   mount --bind /sys /target/sys
   chroot /target
   

   If you've created some other partitions (i.e. /home) mount them here too

7. Open fstab in editor:

   vi /etc/fstab
   

   and update the line with BTRFS partition, i.e.:

   UUID=xxx / btrfs default,subvol=@,ssd,noatime,space_cache,commit=120,compress=zstd:2  0 0
   

8. Finally, setup the bootloader (I assume /dev/sda)

   update-initramfs -u -k all
   grub-install --recheck /dev/sda
   update-grub 
   exit
   

9. Get back to your installer (Alt+F1) and reboot. Ubuntu should boot to your @ subvolume.