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?
I highly recommend, if possible, to backup all the data elsewhere, wipe the drive, removing all the unnecessary partitions like Dell utilities partition, and then reinstalling Windows on the newly blank partitionless drive. If you want a separate Linux partition, then create that separate partition after you wipe it and delete all existing partitions, and before you reinstall Windows.
Your question is unclear, so I don’t know what you’re really wanting to know; but this is the best thing I can tell you with what you’ve given me so far. If you want to elaborate on your question then I can improve my answer for you.