Operating Systems running on your Smartphone and PC

Android is written in Java
iOS is written in C
Windows 10 is written mostly in C

Browser programs are written mostly in C++
A Browser is different to an Operating System. It is a program on your computer (Chrome, Safari, Edge, IE, Firefox) designed to interpret web page commands. 



Web Pages are downloaded from Web Servers
Web Servers ALL send HTML and Javascript commands inside these web pages as a "Front End" to your local browser

Google Search Engine is a program running on its web servers. It is written in Python and C++ and Java

Bing Search Engine running on Microsoft's web servers is written in C++ and C#

Facebook uses PHP, Python, C++, Java, others
Youtube uses C, C++, Python, Java
Yahoo and Wikipedia and Wordpress use PHP
Amazon uses Java, C++, Perl
Whatsapp, purchased by Facebook in 2014, uses Erlang
WeChat is probably written in C or C++ or Java. Cloud Service is perhaps PHP

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