Integrating any review apps to your Google Ads is done by Google themselves. Learn how Product Ratings work and how to integrate with them with this help article from the Google Help Center.
Google is already partners with a few review apps. Go here to see the list of partnered review apps.
If your review app is not listed in the above link, the easiest way to add your review ratings to the Google Ads is by uploading your reviews feed which is available from your review app. Learn how to upload your reviews feed from this Google help article.