What is the difference between software architecture and software design?

Software architecture deals with system-wide structure and high-impact decisions, such as service boundaries, dependency direction, databases, communication styles, and deployment models. Software design also includes lower-level decisions inside those boundaries.

Choosing a modular monolith is primarily architectural. Choosing Strategy to vary a pricing algorithm is usually a design-level decision. The boundary is not absolute, but architecture generally has broader scope and higher change cost.