Booting from a GNU/Linux live distro and using parted we can see the hard disk partition scheme:

Model: ATA kimtigo SSD 512G (scsi)
Disk /dev/sda: 512GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End    Size    File system  Name                          Flags
 1      1049kB  106MB  105MB   fat32        EFI system partition          boot, esp
 2      106MB   123MB  16.8MB               Microsoft reserved partition  msftres
 3      123MB   511GB  511GB   ntfs         Basic data partition          msftdata
 4      511GB   512GB  1258MB  ntfs         Basic data partition          hidden, diag

Here it is the same partition sizes, but in bytes:

Number  Start          End            Size           File system  Name                          Flags
 1      1048576B       105906175B     104857600B     fat32        EFI system partition          boot, esp
 2      105906176B     122683391B     16777216B                   Microsoft reserved partition  msftres
 3      122683392B     510851547135B  510728863744B  ntfs         Basic data partition          msftdata
 4      510851547136B  512109838335B  1258291200B    ntfs         Basic data partition          hidden, diag

The Microsoft reserved partition, according to Wikipedia, should not contain any important data: Microsoft reserves a chunk of disk space using this MSR partition type, to provide an alternative data storage space for such software components which previously may have used hidden sectors on MBR formatted disks.

The 4th partition is flagged as hidden and diag. According to parted documentation hidden means that the partition is hidden to Microsoft operating systems, diag indicates that a partition can be used as a diagnostics / recovery partition.

Once booted into a a GNU/Linux live distro using an USB stick, use the ntfsclone tool (from the ntfs-3g Debian package) to save an image of the NTFS partitions to an USB mounted external hard disk:

ntfsclone --save-image --output /mnt/win11-sda3-ntfs-msftdata.img    /dev/sda3
ntfsclone --save-image --output /mnt/win11-sda4-ntfs-hidden-diag.img /dev/sda4

The small Microsoft reserver partition can be binary dumped into a file:

dd if=/dev/sda2 of=/mnt/win11-sda2-msftres.img bs=1024

The fat32 partitions which contains EFI boot files can be mounted and saved into a zip file:

mkdir /mnt2
mount /dev/sda1 /mnt2
cd /mnt2
zip -r /mnt/win11-sda1-fat32-boot-esp.zip .

In Windows 11 search for the Recovery Drive tool and choose to create a Recovery Drive, an USB stick with at least 16 Gb of space is required. All data on the USB stick will be deleted. This drive can be used to boot the PC and to restore Windows 11.

The efficiency of the process is ridiculous: it required more than one hour to copy less than 16 Gb of data!

Boot into Windows 11 and run the Disk Management tool; right click the Windows C: partition and choose Shrink Volume…. Type the space you want to free up.