If you click the browser share icon, or trigger the browser native share intent, the origin URL will be shared, not the AMP Cache URL. Only if you explicitly copy the URL bar will the AMP Cache URL be shared.
The Signed Exchange spec that AMP has offered sites for a year now allows them to have their own URLs displayed in browsers that support it. In that case, the google.com URL will never be displayed and thus can't be accidentally shared.
All AMP documents on the AMP Cache contain `<link rel=canonical href={origin url}>` and Google recommends that social media prefers the canonical URL. This is useful outside of AMP as there are often multiple URL variants for any article. The sharer and sharee may not ideally get the same version. As an example, a mobile vs. desktop article.