How to get more Google reviews without breaking Google’s policy
Most of the gap between businesses that get reviews and those that do not is timing and friction, not persuasion.
Google stopped showing review rich results for reviews a business hosts about itself, under LocalBusiness or Organization. The reasoning was that self-rating is not independent evidence.
Product markup was never part of that restriction. Reviews attached to a Product remain eligible, along with Recipe, Book, Course and a few others.
Product schema with an AggregateRating, because that is the type that produces stars. It is on by default and there is a toggle.
Correct markup makes a rich result possible. Google decides whether to show one, and it often does not.
The full explanation, with example JSON-LD, is in review schema markup explained.