From my kiddos experiences in 2019 and 2021, you could go back and forth within a section and change answers, but once that section is over, you lose access to it.
They were given blank pieces of paper and a pencil for mathwork.
Have your child bring a sweatshirt because it’s often cold in the testing center.
They will have your child empty their pockets, roll up their sleeves etc every time before entering the testing room, so be prepared for that, especially if you don’t have an English speaking proctor. Also let your child know that the majority of people taking tests will be adults. That was a little intimidating at first!
I believe the screen tells them to take a break, and then he/she leaves the testing room for up to as long as the break is. They can return early if they want, or not take a break at all. For both centers we have used, there is a main room that is separate from the testing room, with chairs, lockers and a bathroom. So during the break, test takers can access food / snacks that they brought (or more likely the parent holds on to these things) and go to the bathroom. Once the test taker leaves the testing room, the proctor gives him/her the current time, to keep track of how long the break is. And then the test taker needs to check in before the time is up, and go back through the emptying pockets drill etc.
Make sure you bring snacks and drinks for yourself too and a good book to read. There was no cell service in either testing center we used, and its a long wait. Some testing centers will allow the parent to leave, but in your situation especially, its probably best to wait there.
Best of luck!