What does Used Space mean?

So, I just made the last step to get my desktop, tablet and phone on 26.0.1. Had to do some shenanigans on the phone as I keep a large library of music there. One odd thing has me scratching my head. In the Finder window when the phone is selected, I get this at the bottom of the window. What does “Used Space” mean?

The phone had been on 17.x, I kinda skipped 18 as I was developing a method to preserve my large music library. It had something like 6.7G free space. After the 26 update and all my music returned, free space is under 2G. Whoops I see a lot of my apps were offloaded. Put a few back on and now I only have 640MB free. Except that “being calculated” disappeared and instead I get this:

So how can I find out WTF this “other” that’s taking up almost 21G on my tiny 64G phone? Contrary to most folks, I dont really use the phone all that much for computing stuff… it’s mostly my outdoor iPod with the occasional text or look at my public transportation, and the very rare phone call.

Oh, could that be what the OS takes up? A THIRD of the storage space for the OS?

See this explanation from 2021 - likely its mostly “temporary” logs and caches that are not efficiently cleared out. The only (somewhat risky) solution seems to do a back up, reset and restore.

Stuff gets weirder and weirder. I kinda forgot I posted this and continued to eventually see that was about 13G for the OS and 9G for the system caches. WAY more than previously. Not real sustainable in a 64G phone. But I had my method of off loading some music (using playlists as I have WAY more than I can get on that phone) then being able to put the exact same stuff back. So starting off (post instaling 26) I had between 600 and 300MB free.

Here’s the way weirder part. An app I use had an update. Tried it, said I didn’t have enough space. Did a smaller prune, saw I had close to 2G free, went to the app. Hmmm, no update notice, so I find out it already HAD been updated. Somehow it must have cued it. OK, replaced the music files I thought had synced out, tried to sync again, did not see any songs being copied like it should have and I STILL have close to 2G free.

NOW it looks like I have all the music I had on it before 2026. Then I had something like an extra 6G free. Remove songs, install 20026, restore songs. 600MB or less free space. Trying to figure out exactly what happened and why is making smoke comes out of my ears, so I’m going to call it a win. Wonder what will happen if there is a 26.0.3, 4 or 5? Not to mention they are saying 26.1 is actually close.