There’s no common standard being established internationally on the hotel star’s rating.
Each country establishes its own star rating process and implemented it through associations or through government departments.
Having said that, the star ratings still serve as a good guideline for one to choose a hotel.
In your case, I believe the star rated reported on Google was submitted by the hotel through Google Business listing.
On top of star rating, I would suggest to check out third-party review sites. E.g.