To my understanding from small MVC applications, you have the frontend which deals with HTML,JS,jQuery,etc and you have the backend which consists of your controllers and models.

However, when I talk to developers from large companies, they often mention having a front end tier and a backend tier. So sometimes, I might hear that they have a frontend with C# and a backend with say Java. Why would any company want a backend and frontend in different languages? Does this help the large website scale better?

When people say that our frontend is built in C#. Does this mean that they are also using a framework for the frontend like .NET and an additional framework on the backend say like Spring in the backend? Or what does it mean that our front end was developed in C#?