storage "other"?

Among other things, with APFS when you have duplicate files (anywhere on your drive) deleting the dupes does not free up the amount of space you might expect; this is due to the APFS "Copy on Write" methodology that doesn't actually duplicate files but creates a duplicate set of pointers to the same data on disk.

It's also possible that APFS snapshots are taking up more & more space. These are system related snapshots created either by macOS when you do an update/upgrade or by Time Machine daily. It appears that Catalina does not remove snapshots once created, although there are techniques to remove them. The daily TM system snapshots are supposed to have only a 24 hour life. Also, note that TM system snapshots are not generally visible to the User and are not the same as regular TM backups.

I may not have used quite the correct technical language here, and if I haven't, please be forgiving.

OK It could be something that happened to me once - I had all this space that was taken up by "nothing"

The MAC has a facility that holds back space should it need it for emergencies and so allocates it - at least that is what happened to me!

What I had to do was fill up that remaining storage for it to relinquish the space in "other volumes"

So you can dump files on your Mac HD and once you get upto the 2.56GB, see if it works and gives you some of that 107.94GB back to you - it maybe that you have to keep duplicating a folder of files to keep it relinquishing all that space (Assuming the first attempt of the 2.56 worked!)

Sounds weird, but I did it and eventually gave me all that "other volumes" space back!

Then you just delete all the files again, leaving the space free!

Bon chance!

Actually it won't give you all if it is also supporting the other HD, but you will know if there is any allocation the way I said, once you fill the storage up - It won't delete any files to make room for you building up the files in storage - so if it doesn't relinquish any storage then this is not the problem!

Make sense?

Is the drive APFS format?

Can you screen shot in disk utilities?

This article may be helpful in finding out what is taking up space on your HD:

Where did all that free space go on my APFS disk?