The basics
https://powersj.io/posts/ubuntu-qemu-cli/
Mount disk images
It is ofter very convenient to mount a FS locally to be able to debug and/or fix a problem with a broken disk.
Pre-requisite to everything: mkdir /tmp/rootfs
To know the format of your disk: qemu-img info disk.img (note that qemu-img can output JSON to automate your stuff)
Raw disk images
losetup -f -P disk.img
losetup -l | grep -v snap # to find the loop device you just created and yeah those snaps....
mount /dev/loopXpX /tmp/rootfs
For anything else (QCOW2, VHD/VPC, etc…)
modprobe nbd
qemu-nbd --connect=/dev/nbd0 disk.img
fdisk /dev/nbd0 -l # to find your partition
mount /dev/nbd0pX /tmp/rootfs