There are a number of ways to build this but the simplest is to build an automation where a new GMB review is a trigger. You’ll get the rating, review text, and some more details as well, and can add them into the CMS.
When I do this I track the source as GMB so it’s easy to identify.
Then you can additionally support your own reviews, perhaps a form + Logic to add them to the CMS, and an internal process to verify & approve it.
The most complex part is the running average, because ideally you’d want that to be part of the dealership’s record for easy access. Honestly your best option there is probably an accrual counter.
In your dealership collection, you’d track Ratings Total e.g. 452, Ratings Count e.g. 100, and Ratings Average e.g. 4.5. Any time a new rating comes from GMB, or is approved by admins, that rating gets added to Ratings Total, and Ratings Count is incremented. Ratings Average is then calculated as total/count and rounded to the nearest 0.5 for star rating display.
Ugly but it’s the easiest to implement and the easiest to re-calculate manually if you ever need to reset the numbers.