Checking the operating system on the site?

Question

JavaScript

Hello, I'm trying to create a dropdown window with a recommendation to download an application, for Android and IOS.
Tell me how, in a mobile view of 768 px or less, create an operating system check on the site, so that a user with a corresponding Android or IOS device would see a certain window with a link to the application?
And the second question, if there are any desktop online emulators for viewing the site from different devices and operating systems to test and debug this function.

Answer 1

To check the OS look here .
From online emulators, I will advise BrowserStack . The trial version has a 30 minute limit.