Tests are online and done during the summer. For the most part kids are appropriately placed but when not, as in my son’s case, they get moved into a different class when the school realizes it made a mistake.
If you want to stress about “preparing” go ahead but the number one thing both my kids learned in BS was to prioritize and not stress about things like this. Kids get so much work they cannot possibly prep the way they have been used to in middle school. I recommend learning this lesson early and not trying to wow anyone with a placement test.