Where are Heroku apps hosted exactly?

The Heroku US region is AWS's us-east-1. EU is eu-west-1.

That makes the US region located in North Virginia, and EU in Ireland.

As the URL you posted, just follow their command in your own terminal shell:

$ heroku regions --common
ID  Location       Runtime
──  ─────────────  ──────────────
eu  Europe         Common Runtime
us  United States  Common Runtime

Then use the curl as they suggest, just change the /regions/frankfurt to the region ID you want (i.e. us)

$ curl -n -X GET https://api.heroku.com/regions/us -H "Accept: application/vnd.heroku+json; version=3"
{
  "country":"United States",
  "created_at":"2012-11-21T20:44:16Z",
  "description":"United States",
  "id":"59accabd-516d-4f0e-83e6-6e3757701145",
  "locale":"Virginia",
  "name":"us",
  "private_capable":false,
  "provider":{
    "name":"amazon-web-services",
    "region":"us-east-1"
  },
  "updated_at":"2016-08-09T22:03:28Z"
}

Here you are: "locale":"Virginia"

Damien got you the answer, I just reconfirm if there is any changes by running all those commands.

The Heroku US region is AWS's us-east-1. EU is eu-west-1.

That makes the US region located in North Virginia, and EU in Ireland.