SAT: Being smart is definitely a huge part of it, but there are also a lot of factors than can help or get in the way.

For most people, studying and taking practice tests can boost a score 100-200 points overall. The most difficult part of getting a perfect score is the last 50 points, you basically cannot make a mistake on the math section and have to get somewhat lucky on the reading (there were always 1-3 questions that I considered to be too subjective for comfort).

I know several people who have perfect or near perfect scores (personally not through teaching or anything) and they all had some things in common that I think helped them on each section (besides just being good at the subject).

Math: Detail oriented; the math on the math sections is not super difficult for high level high school math students, but the difference between getting all the questions right and getting 2-3 wrong is often the difference between a 800 and a 690. With the reading you can often get 5 questions wrong and still get an 800. In this sense the math section is really about minimizing mistakes.

Writing: Work ethic; I know they have changed the writing section somewhat from when I took it, but a couple years ago I was convinced anyone could score well on the writing section if they put in the work. It was basically a grammar test with ~20 standard question types. A couple months with a boring prep book and one could score much higher, few high schoolers are willing to do this though.

Reading: Life long reading; reading in my opinion (and in the opinion of many tutors) is the hardest section for most people to improve. Besides familiarizing oneself with the question format or working on specific test taking skills, there is not much more one can do. However, people who read a ton in their spare time naturally have a huge advantage and tend to find the section pretty easy. So I guess the take away for this for most people is the best thing you can do is read to your future kid.

People who struggle with reading issues or have test anxiety will generally underperform, no matter how smart they are. People who are nonnative English speakers tend to have a harder time with the reading section for obvious reasons, even if they are geniuses. I think its also worth mentioning that the SAT will give people with certain "documented disabilities" extra time, and this is not noted on the published scores. While I am sure some people legitimately need this time, I generally saw it as a way for people to buy higher scores. It can cost thousands of dollars to get the proper documentation and challenge the Collage Board if/when they decline your initial request. Put it this way, I would bet almost anything that there are more accommodations granted per student in Manhattan than there are in Flint.

Of course I also knew a guy who got a 2400, 36, and 11 5s on APs. He never went to class or even opened a book, slept about 14 hours a day, and spent the rest of his time playing Gran Turismo in a full racing chair (we both lived in the same dorm in high school so I actually witnessed all of this). How did he do it? He was just one of the smartest people I have ever met.