The short answer
The architect designs the building, the structural engineer makes it stand, and the contractor builds it.
The architect leads the design and coordinates the brief, drawings and specifications. The structural (and MEP) engineer calculates how the building carries load and serves its occupants. The contractor executes on site to those drawings, with materials and sequencing.
Clear written scopes between the three prevent the most common failure on small projects: everyone assuming someone else checked a dimension, a detail or a rate.
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