A review gives you a score, a risk band and a specialist's notes on everything automation cannot see. Getting more out of it is mainly about what you do before and after.
Before you request one
Fix the obvious automated findings first. A specialist's time is better spent on the things only a person can find. If your scan has fifty unlabelled images and basic contrast failures, fixing those before the review means the notes you get back are about harder, more important things.
Choose your pages carefully. The review covers the pages in that scan. Include your most important pages, anything with a form, and a representative sample of each template.
Reading the audit when it arrives
Go straight to the Expert Notes on failed criteria. Those are the findings automation never would have found, and they are usually the ones that most affect real users. The automated failures are already in your results page.
After the review
Fix what you can, then scan the same pages again and compare. Resolved Issues shows your automated progress immediately. When you are ready for another score, request a new review on the new scan.
Keep a dated copy
Download the Expert-Verified report and keep it. A series of dated reports showing problems found and fixed is the most useful evidence you can have if anyone ever questions your accessibility effort, long after the portal scan is gone.