2) System updates overwrite
/system/ - anything that's common in your current OS and the incoming update will be overwritten, instead of coexisting. However, major updates such as jumping between major Android versions usually introduce a number of added features that will result in either additional files (i.e. not in your current OS) or files that are larger than existing ones, so there will still be a noticeable change in free
I don't have a GS6 myself, so I can't speak for over-the-air (OTA) updates, but I checked the full updates listed on SamMobile, and Android 6.0.x packages are about 1.7GB in size, while Android 7.x are about 2.0GB, so there's approximately a 300MB increase.
Problem is - none of this above should really matter to you, since the changes happen in
/system and will not affect your available internal storage or app installation space (
/data). What you should care as a user is reserving enough space so that the OTA update can be downloaded to and stored on your device.