Shabbat addition

Peace and blessings!
I did not understand the inquiry regarding the Sabbath addition: is the time of the "additional" the same as the Sabbath day and the day is good , or is it a full weekday except that there are certain laws that apply to this time from the time the "additional" is received upon it?
How can it be said that the addition is a real Sabbath (a reference to the Friday Mincha prayer, etc.)?
After all, just as there can be no law that tells me about a dog being a cat, how can a law tell me about a Friday being a Sabbath?
After all, at the end of the day it is a realistic concept.
I understand that I'm probably missing something, but I can't quite figure out the point.