Gatsby pre-builds every page at build time, so tracking has to be baked into the generated HTML rather than added at runtime. The onRenderBody API in gatsby-ssr.js is the standard way to do that for every page in the build.
Yes — onRenderBody runs once per page in the build, whether the page comes from a static file in src/pages or was created programmatically via the createPages API, so the script ends up in every page.
Yes — since the script is baked into the static HTML output at build time, it works the same regardless of where the built files are hosted.
No — gatsby-browser.js controls client-side behavior after hydration, but the <head> content itself needs to be present in the initial HTML, which is what onRenderBody in gatsby-ssr.js controls.
No — Orbit's script tag is unrelated to Gatsby's image and asset processing; it's a small external script loaded independently of that pipeline.