@GetMapping: This method maps GET /hypermedia/employees to this method, the aggregate root.

selfLink in this method points to this method, which is a fixed endpoint.

We flatMap() over selfLink, and then extract every entry from DATABASE, leveraging the employee(String key) method to convert each entry into an EntityModel with single-item links.

We use collectList() to bundle all this into a Mono>>.

Finally, we map over it, converting it into Mono

>> with the aggregate root’s selfLink wired in.