why this happens in c#

Neither of your statements should compile: you are using the Generic List keyword without specifying a Type inside angle-brackets.

Also, even if you had correctly specified a Generic List return Type, both examples would not compile because there is no "return path" inside the body of the method which returns the expected Type.

If you wrote this:

public static List<int> getproduct()
{
    return new List<int> {1,2,3};
}

static public List<string> getproduct()
{
    return new List<string> {"a", "b"};
}

You'd also get an error because you have defined two methods with the same name and identical parameter specifications (in this case: no parameters).

Except for rare cases, like Class constructor overloading, it's a good idea to use unique and descriptive names for methods !

The term "mnemonic" is used to describe a field, property, method, Class name, etc., that helps you immediately understand what it represents ... or what it is for, or how it's used.

And, yes: .NET allows you to re-order the use of the 'static modifier as you wish, but: you can use only one "protection" modifier ! But ... go figure ... this will compile:

internal static protected void doNothing()
{
}

I strongly suggest you study the accessibility modifiers individually: public, private, internal, protected, etc. [^]