How does an API detect I'm requesting from a phone?

I have a paid proxy api which gives me a proxy when doing

requests.get("https://api.proxy.com/get.php?apikey=blabla").json()

If I run this script on a computer I get a proxy, but if I install a python app on my phone and run this script I get 'Forbidden'.

I was just wondering how they detect that I'm sending this request from a phone? prepared_request doesn't help also. I can't find anything except Content-Length and whatever.

Thanks for your help!