Conveniently in Ubuntu Desktop OS’s if you connect an external whole disk encrypted drive, it will ask you to input a password and mount it.

So far so convenient.
However it does not mount all partitions, but ony the boot partition.

This is why you can only see the the kernel, the init ramdisk, grub etc.
To mount the other partitions first use lvscan to scan for logical volumes on all connected block devices.

You can see that one LG (logical group) is currently mounted, the other is not. Using vgchange -ay you can list the active volumegroups to get thier label.

The label is a softlink for a device. You need the devicename to mount the partition. Then check the content of /dev/mapper to see the device name.
Then create a mountpoint and mount the partition.

And now you can access the content of that partition 🙂