Here's a great article from Reedsy, a website that links up authors and freelancers, about what to expect from a fiction edit: https://blog.reedsy.com/fiction-editor/
Story building is like constructing a house. The first step is creating a sturdy foundation. Is your premise strong enough? Is your protagonist complex and easy to relate to? Does he/she have a clear goal with ob-stacles that seem insurmountable?
You've gone through and fixed all you can. Now it's time to hand it to a professional who will tighten your sentences, eliminate redundancy, and increase the microtension. Line editing your book will help to keep your reader engaged.
Not quite ready to do a full-blown substantive edit? Hiring an editor to do an editorial assessment provides you with some professional advice on the state of your manuscript before you're ready to dive in to the heavy edits. Editorial assessments help you know if you're even heading in the right direction.