other system data

I am stumped. I can't afford a new phone right now, and I can't keep my phone updated, nor take new photos. I have an old SE phone from several years ago.

According to the data under General->iPhone Storage there's 15GB of storage used on my 16GB device. System data is roughly 6GB. "Other System Data" is roughly 6GB. I've totaled all the space used by apps, photos, etc. and that comes to roughly 2GB. This tracks (allowing for some amount of round-off error in the reported sizes). But, there's no indication of what "Other System Data" is, how to access it, or how to delete it.

Other messages on this site have claimed that the space reported as "Other System Data" is irrelevant and can be ignored, that it's not really space that's in use. So, if the 6GB of space that is reported as being in use as "Other System Data" isn't real, where is the other 6GB of space that's NOT reported anywhere hiding? And, if it IS really space in use, then how do I delete it??

I've rebooted several times and that doesn't change anything much (I got back about .5GB). I've tried to connect the phone to my Mac but I get a message saying that a software update is needed before it will sync the two. Well, I can't update the iphone so...

How do I get my iPhone down to 10GB so that I can take photos and download the newest OS??