I'm wondering how does Disk Management in Windows shrink the C:\ drive partition when you are booted into Windows (and thus the partition is mounted). If I'm not mistaken, mounted filesystems in Linux cannot be modified in Gparted.
I just shrunk my C:\ drive to 512 GiB to make room for Linux and the operation was done extremely quickly. If it matters, the Windows partition is NTFS and there's a few partitions both before and after it (recovery and Dell utilities partition).
From this question, I realize that there's unmovable system files in the middle of the partition, but how does the OS know that there's no files past that point? Or am I thinking about this process completely wrong?