The short answer
Architecture shapes the building envelope and structure; interior design shapes the spaces, finishes and furniture inside it.
Architecture deals with the building itself: siting, structure, envelope, circulation, services and statutory approvals. Interior design works inside those boundaries — layouts, finishes, joinery, lighting and furniture.
The two overlap heavily on residential projects, and many practices offer both. On a new build it is usually cheaper to decide the interior layout while the architecture is still adaptable, rather than after walls are cast.
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