Your C drive is full and Explorer shows nothing big. Where it went

You have added up Documents, Downloads, Desktop and Pictures and it comes to a few gigabytes. Windows says the C drive is nearly full. Nothing you can see accounts for the difference, and running Disk Cleanup finds a few hundred megabytes.

This has a short list of causes, and Explorer hides every one of them.

The seven, in the order worth checking

See the whole drive first

# PowerShell, in the folder you want to look at
Get-ChildItem C:\ -Force -Directory |
  ForEach-Object {
    $s = (Get-ChildItem $_.FullName -Recurse -Force -File -ErrorAction SilentlyContinue |
          Measure-Object Length -Sum).Sum
    [PSCustomObject]@{ GB = [math]::Round($s/1GB,2); Folder = $_.FullName }
  } | Sort-Object GB -Descending | Select-Object -First 15

That is slow, because it walks everything, and it is honest, which Disk Cleanup is not: Disk Cleanup only reports categories it knows how to remove, so a 30 GB folder it has no opinion about is simply absent from its list.

The two that surprise people most

The hibernation file is sized against your installed memory, so a machine with 32 GB of RAM can be holding 12 to 25 GB for it, and it exists even if you never hibernate. What hiberfil.sys is and how to remove it covers the trade.

System Restore reserves a percentage of the drive by default and fills it with shadow copies. It is genuinely useful and it is also invisible: nothing in Explorer shows it, and the setting that caps it is buried in a legacy dialog.

What Windows shows you, and what it leaves out

Settings, System, Storage is the built in view and it is worth opening first: it finds temporary files, the Recycle Bin and large unused apps quickly. What it will not do is show you a folder tree, which means anything that does not fit one of its categories lands in a bucket with no way in, exactly like System Data on a Mac.

The order that gets the most space soonest

1. Empty the Recycle Bin, on every drive rather than just C.
2. Settings, System, Storage, Temporary files. Tick everything except Downloads unless you have looked at it.
3. Remove Windows.old through Settings if it is still there.
4. Turn hibernation off if you never use it.
5. Cap System Restore rather than disabling it.
6. Run the component store cleanup with DISM.
7. Then, and only then, start looking at your own files.