For the backend web API you can use ASP.Net or Azure functions depending on what you are trying to do.
If you also use Blazor, you can do front and backend in C#. The dB will still be it’s own thing, but odds are you will be able to get a library which will allow you to write most of your queries in C#, such as Entity Framework(EF). EF uses Linq to create an expression tree which gets converted behind the scenes into SQL. The creation of the tables etc in the dB can be done with migrations in EF too.
So if all you want to use is C#, there is potentially the possibility to do so. It will depend what you want to do exactly as there are some limitations in some of those solutions, which may mean you need to write SQL yourself, but EF does allow for it.
So with back-end development, you’re free to use just about any language you want. In fact, a lot of these major tech companies (FAANG) use multiple languages for different back-end engineering projects.
That being said, a lot of jobs requires certain frameworks for back-end, so it wouldn’t hurt to be knowledgeable in things like Azure, .net, AWS, etc.
Just keep in mind, depending on the language you pick, you may be limited to the framework flexibility you have accessible to you, and things may not mesh the way you want them to. If you’re planning on getting a job in the field, I recommend going on Indeed and seeing what requirements are for back-end engineers and full-stack engineers so you can get an idea of what companies around you use, and you can target your back-end development to line up with the job you’re looking to get.