"Other" just means "stuff that doesn't fit the few specific categories I list", so I tend to regard it as a useless label.
One possibility is that something is storing a lot of data in the /data/data volume. This is what's used by apps to store their private data, so won't be visible to file browsers or similar. But 10s of GB does sound like rather a lot (it won't be 100 GB, because some of your 128GB is used by the system for the ROM, device firmware, recovery module, bootloader and all of the other gubbins that the phone needs to work, but even with Samsung, who tend to build big ROMs and be even more generous with the space they allocate to them, it won't be close to 100 GB - though it will be well over 10GB). So there's something fishy here.
A couple of things to try:
1) Do your phone's Apps settings (may be called "App Manager" on a Samsung) let you list apps sorted by size? That will be the quickest way of seeing whether there is something like this going on.
2) Let's try to get some information from apps that I know and trust. "Storage Truth" is a very simple little app (written by one of our moderators) that will, on an unrooted device, tell you the actual size of your /data partition (which is the bit of internal storage not used by the system) and how much is used/free. You may think you know this, but actually neither of the screenshots you show says 100GB is being used, just that about 27GB are used and there are only a couple left, so I'd like to get explicit confirmation on this point (I'll explain below). The other is "DiskUsage", which gives a pretty detailed graphical view of what is using the bits of your storage it can read: it won't tell you anything about system usage, but should let you spot if any app is a bit data user.
Now, the reason I wanted to confirm that there are actually 100GB used as opposed to only a couple free is I want to be absolutely sure that your device actually has 128GB. Because there's one other possibility that occurs to me (and apologies, but remember I only know what you include in your post so have to consider all possibilities): are you sure this is a real S20? If you bought it through your service provider or direct from Samsung then it will be. If you bought it second hand, as an import, or from a seller on an online market (including Amazon Marketplace), then the possibility that it isn't is a more serious one. And fakes often go to great lengths to make the phone and its user interface look like the real thing, and will program the system settings to lie about the device parameters, but they usually cut corners seriously, including installing less storage than they claim. So just to be certain I'd like to consider and eliminate that possibility as well.
