Galaxy S6 32GB system memory issue

[Rog21]

Hello all.
I was having some issue with Galaxy S6 slowdown so I did a factory reset. I am on Android 5.0.2.
After the factory reset was complete, when I tried to install any app, it would say it doesn't have enough space. When I checked the storage, I was shocked that out of 32 GB, 31.02 GB is reserved by system memory.
Anyone know how to fix this? No matter how many times I factory reset it, it still takes 31.02 GB. Please check the attachment.

If it's an issue with Android, can I upgrade to Android 6 and if there is a workaround for that since when I check for auto update, it says, I have the latest version.

 

[Rukbat]

There's something wrong there. Since you have nothing on the phone to lose, flash the stock ROM - [Samsung] How to flash Stock ROM via ODIN.

And see [Guide] Storage is Missing to get a true idea of what's happening in storage.

 

[Rog21]

Can you please point me to the firmware for my phone?
Since I have 5.0.2, which firmware should I install?
My phone is rooted and has bootloader unlocked.

 

[Rukbat]

1. If you want rooted firmware, go to XDA Forums. Samsung firmware doesn't come rooted.

2. It depends on the exact model number of the phone - G920F, G920FD, G920FQ, G920I, G920A or G920T?

Then it may depend on the country and/or carrier. Go to https://www.sammobile.com/devices/ and put in the device number (or put in s6 and choose the device).

 

[Rog21]

My phone is ATT G920A.
I tried searching for the firmware but there are too many options and not sure if I am in the right direction.
Can you please share the link to rooted firmware and do I go with Android 6 or 7?