As I understand MVC,MVVM,SOA,Onion are an architecture, but I really got confused. I think this is not correct that I assume all of them define as the same Architecture concept. I want to know what is true about architecture...
This question is very broad and it does not have a commonly accepted answer. It will probably be downvoted and closed sooner or later. I will try to answer it anyway.
An architecture is a property of a specific piece of software. MVC, MVVM and SOA are architectural patterns that can be implemented by different pieces of software.
There are many definitions of architecture. My favourite one is:
"The things that are important and will be hard to change later."
This definition has been phrased by Martin Fowler but I am not sure if he was the first to come up with it. I like it because it makes clear that it is a bad thing to have more architecture than absolutely required, especially in a world where changes are likely to occur and the future is hard to predict.