How to Reclaim 18-20GB SSD Missing Space Windows 11??

Go to settings-System-storage.  You can see the main groupings and you can expand them out to see what is taking space.

If you have a 512GB drive, is it really necessary to limit Windows to only 100 GBs.  Moving default Windows folders to unauthorized locations may also hinder future upgrades.

As far as removing the Recovery Options, I would not suggest that since it may interfere with future upgrades.

I suspect you're not taking into account protected system files (e.g. hiberfil.sys, pagefile.sys, swapfile.sys). "Show hidden items" in File Explorer does not show these, and they may take up close to the discrepancy you cite. There's also some overhead for filesystem metadata (the file pointers that make up your folders, etc.) which is something like half a percent of the filesystem space.

Like @Saltgrass I'm really surprised your C: partition is only 100GB. Was this intentional? What other partitions are on your SSD? (You can use Disk Management, part of the Computer Management app, to check.)

What might make sense (if not too much trouble) is to reinstall Windows 11 from Microsoft installation media. This creates only one recovery partition of less than 1GB. You should still be able to install all the Dell-specific drivers etc. from SupportAssist, Windows Update and the Microsoft Store. Before doing this of course you should have your files backed up, and you could also image the drive (perhaps with Macrium Reflect?) so you can restore to where you were if things go south.

Note that your 512GB SSD will appear in Disk Management and other tools as 476GB. This is because storage vendors measure storage differently from anything else in computing (unfortunately). It's hard to explain but it's the truth; see here for more.