Why is my 9 year old Sony DSLR better than my 2 year old Nikon DSLR?

Why is my 9 year old Sony DSLR better than my 2 year old Nikon DSLR? - 1

First of all, "better" is a highly generalized and relativistic term. Better at what? I'm going to assume image quality, though you could mean build quality, ease of use, ergonomic etc.

Then we don't know the model number or sensor size, or lens you are using. All things that contribute to image quality.

Perhaps it is nothing more than the time you spent with the Sony, having greater familiarity, including more familiarity with post processing.

In general, many things going into current model DSLRs are vastly improved over the last 9 years. But that doesn't mean everyone is going to get better photos with a newer camera. I would suggest going to a website called preview and find your camera and lens models there and find out about their strengths and weakness' to understand better what you are asking.

Sony is better Brand than the old nikon

As you have not said which models you are referring to, or what your criteria or evidence is frankly your question is nonsense.
It is quite likely you don't understand the equipment you own.

Because Sony spend more time on making good cameras, than telling the world how great they are.

Oh… It is?

Maybe it is, maybe it is not. With the total lack of details in your question, there's no way to give a proper answer. If your question is an example of how you pay attention to details, then I have very slim confidence in your assessments.

Indeed "better" in what way?