What actually decides it.
Profile completeness and activity. A profile with correct hours, current photos, answered questions and recent posts outperforms one that was claimed and abandoned. This is unglamorous, continuous work and it is most of the job.
Review volume, rating and — importantly — recency. A business whose most recent review is from last year reads as a business that used to be good.
Citation consistency. Your name, address and phone number have to agree across the directories that feed the map. Inconsistencies prevent the signals consolidating, and they usually accumulate from a move, a rebrand or a phone change that was never propagated.
Proximity to the searcher, which you cannot change. It is worth knowing this, because it explains rankings that otherwise look inexplicable and it is why an "average position" across a market is a nearly meaningless number.
Claim all three platforms, not one.
Google gets the attention. Bing Places matters more than its search share suggests because it feeds other surfaces. Apple Business Connect decides how you appear in Apple Maps and to Siri, and it is frequently unclaimed — which means a meaningful share of iPhone users see whatever Apple inferred about the business, sometimes an old address.
Do not build forty city pages.
Templated location pages with the city name substituted are a recognized pattern and they can drag down the rest of the site. If a page has nothing true and specific to say about that location — the work you actually do there, the conditions particular to it, the customers you actually serve — it should not exist.
One properly managed profile and reviews that keep arriving will beat forty thin pages, consistently.