Where to find the code that redirects requests on the static page URL to the home page

First off, the behavior you describe is not a redirect but a URL rewrite.

That being said, there’s a WordPress option called page_on_front that is set to the page ID of the static page to be used as a front page. This option’s value is only relevant if the option show_on_front is set to page (that happens if you set “Front page displays” to “A static page” in the “Reading” panel).

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Depending on the page you select from the dropdown below that radio box, page_on_front is set.

You can however also set options programmatically. The following approach is untested and just a quick idea off the top of my head, but it should be possible to simply set this option to the sticky post’s ID and be done with it:

$sticky_id = // set to post ID you want to display
update_option( 'page_on_front', $sticky_id );

Another approach (if this first idea fails) might be to set the front page to a regular static page, assign that page a custom template, leave the page content empty and incorporate a custom loop that grabs the latest sticky post in said page template. That ought to work fo’shizzle.