A Google review removal company assesses a review against Google's published content policies, builds the evidence that the review breaches one of those policies, and then lodges and manages a removal request with Google on your behalf — including appeals when the first report is rejected.
What no provider does is remove the review itself. Only Google can take a review down, and Google makes that decision on its own assessment. What you are paying a specialist for is policy knowledge, evidence preparation, correctly worded escalation and persistence — the parts of the process that determine whether a legitimate complaint is actually considered properly.
That distinction matters when you compare providers: anyone who implies they control Google's decision is overstating their role. The honest version of the service is "we know how to put your case to Google properly, and here is what you pay if it doesn't work."