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It sounds a bit plain and boring, but if you're just wanting to learn the basics of C# then you could refer to the official Microsoft docs:
They're quite basic, but do offer some fundamental knowledge. If you're after more theory such as architectural patterns, I'd say you're better off looking at resources that are language-independent
I find Refactoring Guru to be an invaluable resource. Start with the design patterns: https://refactoring.guru/design-patterns/catalog
Then take a look at the Refactoring: https://refactoring.guru/refactoring/catalog
It's great for theoretical knowledge of SOLID principles etc, if you dig deep it does have C# specific implementation examples.
There's also an eBook version you can purchase if you're so inclined