In Termux (outside VM), run:
qemu-img resize debian-12-nocloud-amd64.qcow2 +30G
Change debian-12-nocloud-amd64.qcow2
to the real file name. +30G
indicates increasing 20GB disk image. You can adjust the size as needed.
Inside VM, run:
sudo apt update
sudo apt install parted e2fsprogs -y
sudo parted /dev/sda
print
fix
resizepart 1 100%
quit
sudo resize2fs /dev/sda1