For me, personally, heavy use of the => operator (which happens to coinside with my main complaint of a lot JavaScript code and anonymous functions). You can avoid it, but is pretty standard.

Very specifically I also looking into JWT authentication in ASP.NET Core and found the whole thing really tricky to wrap my head around. That's more of a library, but I think many of the usage examples ends up being a bunch of spaghetti code.

Wait, what? The => operator is just for lambdas, short-form returns and switch expression arms, which is as common as it gets!

Have you never worked with any other language which lets you do these?

    var say = (string s) => Console.WriteLine(s);

or

    struct Lease(DateTime expiration)
    {
        public bool HasExpired => expiration < DateTime.UtcNow;
    }

or

    var num = obj switch
    {
        "hello" => 42,
        1337 => 41,
        Lease { HasExpired: false } => 100,
        _ => 0
    };

You'll see forms of it in practically every (good) modern language. How on Earth is it confusing?

Authentication is generally a difficult subject, it is a weaker(-ish) aspect of (otherwise top of the line) ASP.NET Core. But it has exactly zero to do with C#.