For some reason, my internal storage says there has been 15.88 gb used out of 16.00 gb. When i go to the files in internal storage and I select all the files, when I press details, it says all the files add up to 728 mb. This is really frustrating and sometimes i cannot delete files that I do not need.
Most of that is the Android Operating system with it's default app set plus the apps Samsung bundled on your hardware. If you have any TV catchup apps installed make sure you delete any downloaded programs you have already watched. Use Google Files to safely clear out unnecessary cache files and some unused applications.
If your device only has 16GB internal storage you are going to need to leave the Micro SD card in permanently in 2021 for any photos, downloads or other personal files. 16GB is not really sufficient any more, even if you did a device reset you'd still find 13-14GB of it unusable to you