Using a Joomla Extension
There are currently very few Joomla extensions that help with displaying Google Reviews. Examples include Google Business Reviews (from €34.95) and Google Reviews Pro (from $39.00).
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
You can add a "Show More Reviews" button to link directly to the reviews page on Google. For a direct link to the reviews, search for your business in Google, click on the "View all Google reviews" link in the business details on the right hand side of the page and use the url in the address bar.
You can also add a "Write a Review" button to make it easier for website visitors to leave you a Google review. To obtain the correct link, log in to Google, search for your business in Google and click on the "Get more reviews" in the business details on the right hand side of the page to reveal the link.