Why is space not being freed from my MacBook Pro after deleting files?

Windows has its own issues, i don't think switching back to Windows would be a wise solution.

When you delete files from an external drive, they’re often moved to a hidden .Trashes folder and may not be fully removed until you manually empty the Trash or use Terminal commands. That explains why erasing the external drive restored space—because it cleared everything at the filesystem level. For internal storage, this shouldn’t normally happen, especially on a MacBook Pro with that kind of hardware. Sometimes, system processes like Time Machine local snapshots, cache buildup, or Spotlight indexing can hold onto disk space longer than expected. Reinstalling macOS forces a cleanup, which is why it likely worked for you.

But no, switching to Windows isn’t the only solution. Instead, you can try using Terminal to manually clear local snapshots (tmutil listlocalsnapshots /), check storage with Disk Utility, or use a third-party tool like DaisyDisk or OmniDiskSweeper to see what's taking up space.

Check for large files in System Settings > Storage > Documents, as shown in the image below. Delete unnecessary files from here to free up some space on your Mac.

How are you determining the available space?

Notice that it also depends on what you delete on an APFS volume: a duplicate file takes no space and deleting a duplicate file frees no space.