the cut off is the lowest atar received by the person wanting to enter a course, so for example you want engineering flexible @UNSW -91atar, 91 will be the lowest atar received to enter that course, similar to a deadline, if you are a few points of you may not get into the course depending on student count. In your case you need 80 atar and you get 80 atar, your availability is depending on course student count/bonus points. But mostly getting the deadline of a course cut off is usually guaranteed in most cases.
The chances of you getting in is pretty high-but you have to not only consider student count, but more specifically, students who applies for bonus points and various other schemes which means if there is a lot of students that does it, you might just scrape in, and atar cut offs actually vary from year to year, so it is always better to have an atar higher than last year's cut off to maximise your chances of getting into a course. But I wish you all the best in getting into the uni course you want
Nope cut offs published are last years, depends on demand and supply this year. The only exceptions are conditional early offers and schemes like UTS have with their "gauranteed entry" (usually higher than the cut off ends up being but gaurantees you a placement).
Yeah, supply/demand can fluctuate the cut off each year. Some courses have been known to fluctuate +/- 3. Also some years, if there is a big change in how unis decide to run the course(i.e when UTS decided to offer more places in their journalism degrees)-it was 97 in my sister's day, it's now 89.
Some course have fixed ATAR entries though (like Adv Science is always a flat 95 at most instituions).
If you are in some kind of scheme i.e early entry, a guarantee cut off (like E10, KLD, SEAS melb cut offs, other equity cut offs)-early entry you are already in the course, and the others are all a form of provisional entry that you will absolutely get in if you meet whatever X score they specify (though sometimes the criteria can include a Band X in a subject area).