I can't speak for all of its applications, but I can say that it is a very very widely used language.
In fact, I attended a talk earlier today at a conference and one of the first statements made by the speaker was that despite hearing about Python, Java, JavaScript, and others, C++ is still a huge staple.
C++ is used for a lot of "back-end" type of stuff. JavaScript and front-end frameworks are used very often now in the presentation layer of applications, but a lot of the "heavy lifting" algorithms which are responsible for application functionality are written in C++.
For example, say there's a service which offers some heavy data crunching on a business's data, well, chances are front-end components like the user interface, web server, and database are using a web stack like JS/Python/SQL or NoSQL... But the core algorithms performing the work are likely written in C++. Why? Because C++ allows for fine-tuned control of memory and is very performant when implemented properly.
Video games, operating systems, drivers, image and audio processing, antivirus, and more all commonly have a lot of C++ components. Also, many larger businesses will write their own custom back-end stuff in C++ for high performance and to make the overall user-experience better.
GUI applications can and are also still written in C++ as well using libraries like Qt or Microsoft's C++ GUI APIs.
C can do all of that too, but C++ is more common these days. Higher-level languages like Python and JavaScript have the advantage of memory safety, cross-platform compatibility, and easier code constructs, but C and C++ have the advantages of purpose-built power. Think of an all-in-one wash and wax for your car versus dedicated car wash and a premium wax. The second option takes longer and more manual labor, but it is much more fine-tuned and can offer a better result in the end when done right.