How can one customise library search results from the header search box?

Question

As announced by Microsoft and discussed in this answer, the search box in the command bar has been removed from document libraries, leaving the header bar as the only place with a search box.

When launching a search from this bar, the search results appear in the same view (rather than the Microsoft Search view, which only appears if the "Expand search to all items in this site" link at the bottom is clicked"). The page that's opened appears to be the view with a query string appended - e.g. https://sitename.sharepoint.com/sites/groupname/Test/Forms/AllItems.aspx?view=7&q=searchterm

I'd like to customise the results to promote particular content types.

I've tried so far:

• adding a new result source and setting it as the default in the Microsoft 365 Admin Center
• adding a query rule in the site settings

Nothing has had any impact so far on the results that appear. What settings need to be altered to have an effect on these results?