The documentation for this directory is at:
web_tests/external/wpt/config.tmpl.json is a configuration file template for overriding wptserve‘s default routes and ports. wptserve’s default ports conflict with those used by httpd for legacy web tests, so the configuration file remaps them elsewhere.
Note that the contained filesystem paths are relative to blink/web_tests/. At runtime, run_{web,wpt}_tests.py will resolve them to absolute paths so that the serving behavior doesn't depend on its working directory.
When changing the ports for new servers, make sure to also:
WPT_HOST_AND_PORTS in //third_party/blink/tools/blinkpy/web_tests/port/driver.pyWebTestContentBrowserClient::GetOriginsRequiringDedicatedProcess