Glad to hear you’re enjoying GP and good to see the Site library being used.
Sites are not child themes. They however will not ‘blow up’ when GP updates 🙂
They are an imported package that includes all of the Theme settings (which modules are loaded, all customiser settings and any Hook content), the plugins and there settings, the associated media / posts and pages and sometimes quite a bit of CSS that is in the Customiser > Additional CSS
This varies from user to user.
Me personally i keep all theme elements, blog layout and woocommerce in GeneratePress.
I disable the Elementor global colors and fonts so i can control most of them through the Customiser.
Always use the Default template and if i need an empty canvas use the Disable Elements and Layout options in GP. This way i keep the Hooks and ability to use Page Headers.
If i need to add elementor based content globally or conditionally i will use GP hooks or widget areas to add them. And really keep Elementor to the page content only.