Full disk images contain the bootloader and partition table. To mount an image file, view the contents of the file, then specify a byte offset that points to the beginning of our linux root partition. We're essentially telling our mount command start reading the IMG file from somanybytes from the start.

Run fdisk -l file.img.

Here, you want to make note of the value under the "START" column. Basing it off the size of the paritions, I can assume my root partition is the one that starts at 532480 bytes in.

[...]
Units: sectors of 1 * 512 = 512 bytes
[...]

Device                                    Boot  Start     End Sectors  Size Id Type
2021-05-07-raspios-buster-armhf-lite.img1        8192  532479  524288  256M  c W95 FAT32 (LBA)
2021-05-07-raspios-buster-armhf-lite.img2      532480 3661823 3129344  1.5G 83 Linux

Now I have to multiple the start sector (532480) by the sector size (512). I'm using the bash to do the multiplication from within my mount command.

mount -o loop,offset=$((512*532480)),rw ~/2021-05-07-raspios-buster-armhf-lite.img /mnt

-o Pass options to mount

loop Creates a psuedo block device to mount the image file to

offset=$((512*532480)) Sets the offset to the sum of 512*startSector

rw Enables reading and writing to the mountpoint

The drive should be mounted. If your root partition isn't the last partition, you'll want to google how to specify the start/end byte offset for mount.