Basically, ATAR requirements for a uni course are mostly based on supply and demand for the course - how many people apply for the course, and how many places there actually are in the course. Each year, there are varying amounts of students applying for a course, and only a certain amount of offers that can be made for a course (some courses have a strict amount of places, some have only a general sort of range of places usually offered).
The reason courses like medicine, law, dentistry, etc. have high ATAR requirements is because of the sheer number of people applying for a limited number of positions. The cut-off gets so high, so more people see it as super prestigious due to the increased difficulty of entry, and then want to apply to study that course, so then there's more applicants, and if my explanation just made sense, maybe you can see that it becomes a bit of a cycle.
When you get to uni, they don't care what your ATAR was, just that you are in the course. This was actually the first thing said to my cohort on the first day of uni - paraphrasing, it was essentially: "You're in the course now, we couldn't care less about what your ATAR was, or whether you had to do another course to get into this one, or anything else. You're all in this course, so you're all equal".
Generally, most employers don't care which uni you went to. What they care about are your skills, etc. After all, what good is an employee going to be if they're useless at their job?
I think there are a few exceptions, the main one I've heard about is law (law students, feel free to correct me if I'm wrong) - some firms might have a preference for graduates from a specific uni, etc. But in the majority of courses, they don't care. What they care about on your resume is the degree, not the place it was obtained at.
I think this is particularly relevant in careers that are regulated by a professional body and require a specific course that is accredited with that organisation to be completed (speaking from experience, this is seen a lot in healthcare). Because all the courses have to meet the accreditation requirements, they're very similar, so you see even less bias in the workplace over the place the degree was obtained at, if any.
The only reason a course with a lower cut-off may look bad is because of people's perceptions. It doesn't actually make it any less of a legitimate course, or any easier compared to other courses. If a course with a lower cut-off appeals to you, don't be afraid to apply for it!
(I've also heard from some sources that ATAR cut-off is partially to do with the "academic standard" unis believe is necessary for the course, however this is something I've only read in a few places and personally, my careers advisor at school never mentioned it, so if it's legit then it's not a big factor. If someone has any more info on this, please chime in)
I hope that this has helped somewhat, if you have any more questions, don't hesitate to ask! I would also recommend having a chat to your careers advisor at school, they can walk you through so many things and can be one of the most helpful resources for you in VCE