Hello, I found in the FAQ your solution to change the size of the title review. You give a snippet to add in the functions.php file of our theme. This works fine but what about the next theme update? We will loose this edition. How can we make this permanent?

Use the Code Snippets plugin, or add the code to the functions.php file of a Child Theme.

Also, we have translated the plugin with Loco Translate. You recommend “custom” that use: languages/loco/plugins/ is this safe from any updates (plugin, theme, WP)?

Yes.

Lastly. Does having a website based reviews plugin have any effect on Google reviews. We still have to see how managing locale reviews can help us promoting online reviews like Google or Yelp for example?

Website reviews are separate from the reviews posted on Google Maps (Google Reviews).

Enabling the rating schema in Site Reviews for rich result snippets in Search results and using a plugin like RankMath Pro to generate schema for your website may help your page ranking in Google. Note: Site Reviews is not a schema plugin, it simply provides the rating schema generated from your reviews. I recommend to use RankMath Pro and enable the integration in the Site Reviews settings, but it will take some work to configure your RankMath schema for best results.