As @megavolt said in the 5.81 gb memory usable out of 8gb - #4 by megavolt he has nothing to advice to you currently. Thanks to him.
How do you think are you alone on the planet with this effect? If you let what probably it is not, than to which point your search and investigations brings your thoughts to? What are possible sources of the that hardware RAM usage? For me it looks like some hardware took that RAM, probably it is manufacturer related question how to setup HW of that notebook to let you use a bit more RAM for an OS level usage. Depending on my experience I am sure that if you install other OS, it will be near the same RAM remaining for OS.
I have iGPU only and configure it in UEFI (so-called an improved version of BIOS) to use as many RAM as it can (1024 MB in my case).
In case of my HW I have 900 MB reserved by some HW, all other RAM is avail. to use in OS. I did not investigate by which HW was reserved that 900MB.
Try to search and to try, post what you find and even tried here. May be for someone it will be a clue to advice somewhat else to try also.
Also, if you will unhide your system information your case could be more understandable by others: [HowTo] Provide System Information
even for you: you can try to do more specific search by your motherboard/notebook model and other HW you use.