Looking at the docs, it doesn't appear that you can use the Google My Business API to get reviews on any business location unless you have an account id for that business.
Is there another way to do this without the API or with another API?
I'm guessing web scraping could work, but it may not be stable.
For locations that you don't manage, you can use the Places API to retrieve some reviews, however I'd imagine that would become costly, depending on how many locations you want to look into.