C# is a language designed by Microsoft and is often used in business applications and is supported on many platforms, including mobile and IOT.

It can be used on Linux, Apple, IOS, Android and can even run on a raspberry pi (ARM).

The dotnet (Framework part is the old windows only version) is just the runtime engine of C# / F# and Visual Basic.

So to answer your question, if it is worth it to learn, it's a good and easy language to learn object oriented programming and has very good tooling available, you can either use vscode (free) or jet brains rider (payed) on Linux or Mac and for windows you also have the option for visual studio community edition (free).

But in general pick a language and stick with it for a while, whichever language you pick to start with doesn't matter that much. The art of programming is almost the same in every language and syntax doesn't matter much.

https://dotnet.microsoft.com/en-us/ https://dotnet.microsoft.com/en-us/learn