In RepU, open your widget, choose light or dark and left or right, then click Copy next to the embed snippet.
In the backoffice, go to Settings, then Templates, and open your master layout template. Paste your code just before the closing </body> tag.
Need more detail? See the official Umbraco guide.
Click Save, then open your live site to see your Google reviews.
Check your live site in a private window, not the editor or preview. A domain-locked widget only shows on your own domain.
In production mode you cannot edit templates in the backoffice, so edit the template file and redeploy.