There is a link on the Extend admin page,
Add contributed modules → to extend your site's functionality.
This takes the user to the drupal.org Download & Extend page. It would be better to instead take the user to the Browse modules admin page.
When Project Browser is enabled, override the linked text "contributed modules" to direct to /admin/modules/browse
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I've come up with a preprocess_page hook to target the 'extend' page but I can't figure out how to replace the specific 'contributed modules' tag. Posting the patch with my work so far so that someone else could fill in the gap.