Using a Joomla Extension
There are currently very few Joomla extensions that help with displaying Google Reviews. One example is Google Business Reviews (from €25).
Inserting Code Manually
You can find instructions for manually inserting the appropriate code into your website e.g. see How to display Google Reviews on a Joomla website from Nicky Veitch at Websolutionz.
A Quick Option
Another free quick option, not involving a Google API key, is to manually recreate a few positive reviews with appropriate links for website visitors to read more reviews / leave a new review. You can see an example on Kamron Brooks website at https://skyspider.com.au/#reviews