what http request header is used to denote what uri linked to the resource being requested?

The HTTP request header you’re looking for is the Referer header. In plain terms:

Referer is the header that tells the server which URI (page) linked to the resource being requested.

So if page A at https://example.com/page.html includes an image from https://cdn.example.com/img.png, when the browser requests img.png, it will typically send:

http

GET /img.png HTTP/1.1
Host: cdn.example.com
Referer: https://example.com/page.html

That Referer value is the URI of the document that contained the link to the requested resource.