Apple

Altstore is not available in the usa. Make's sense, fascist capitalist company. However I did find this

In the U.S., the Department of Justice filed an antitrust lawsuit against Apple last year, but the case is ongoing and alternative app stores on the iPhone remain unsupported there.

Thanks to macrumors.com

In case you didn't know, Android allows you install apps outside of the google app store. Freetube as an example. Apple typically doesn't let you install anything outside of their app store which usually extorts the authors for money.