The one thing to know about HTML
It is a script tag and a container div, with no dependencies and no jQuery, and it does not block rendering. The one thing that stops it is a content security policy on your own server — the browser console will say so explicitly.
Where reviews belong in a HTML site
Exactly where you paste it. For floating layouts the position is set in the widget rather than by where the code sits.
Full-width sections suit carousel and grid layouts; sidebars and narrow columns suit a badge. Floating badges can be restricted to particular pages and to desktop or mobile.
Reviews and search results
The widget outputs review schema markup automatically. Whether Google shows a star rating depends on what the reviews are attached to — worth reading before you expect stars, because for a business reviewing itself the answer is usually no.
What does help either way: reviews on the page are content, and a page that answers what customers actually say is a page people stay on.