What do we mean by a software architecture? To me the term architecture conveys a notion of the core elements of the system, the pieces that are difficult to change. A foundation on which the rest must be built.
Once you have an idea of what you're going to build, then you can start thinking about how you're going to build it. Of course, you've already been thinking about this from the very first requirement, but now you have permission to do it. Now we begin to delve into design.
There are really two levels of software design. The level we normally think of when we're writing programs is usually called detailed design. What operations do we need? What data structures? What algorithms ...