Ubuntu server only uses a part of the available disk space

Solution 1:

It looks like the disk was split in three partitions:

The fist partition is labeled and formatted as fat32 and mounted on /boot/efi

The second partition is labeled and formatted as ext4 and mounted on /boot

The remainder of the disks makes up sda3 and is not labeled as a specific file system.
It is managed by the linux logical volume manager. Check with pvdisplay. Physical storage managed by Linux LVM gets managed in a volume group and can be assigned to none or more (what’s in a name) logical volumes. Those logical volumes can in turn be formatted with file systems and mounted.

Possibly not all physical storage is assigned to logical volumes. Check for “free PE” in for instance the pvdisplay and vgdisplay output

Unassigned storage can be used to grow existing logical volumes and file systems or be used for new volumes and file systems , depending on your needs and preferences