What is the difference between static methods in a Non static class and static methods in a static class?

The only difference is that static methods in a nonstatic class cannot be extension methods.

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In other words, this is invalid:

class Test
{
    static void getCount(this ICollection<int> collection)
    { return collection.Count; }
}

whereas this is valid:

static class Test
{
    static void getCount(this ICollection<int> collection)
    { return collection.Count; }
}

A static class can only contain static members.

A static method ensures that, even if you were to create multiple classB objects, they would only utilize a single, shared SomeMethod function.

Technically, there's no difference, except that ClassA's SomeMethod must be static.

If you have a non-static class containing only static methods, you could create an instance of that class. But you can't use that instance meaningfully. NB: when you don't define a constructor, the compiler adds one for you.

A static class does not have a constructor, so you can't create an instance of it. Also the compiler gives an error when you add an instance method to it (where you meant a static method).

See also docs.