Download as in run the app locally or download a design?
If the former, that's not currently possible.
If the latter, you can use the Download button on the harness builder (top floating button menu) to choose an available format for an engineering drawing, diagram, or BOM or choose JSON or WireVIZ YAML; see the Export and Documentation section here: https://splice-cad.com/#/tutorial
Download as in run the app locally or download a design?
If the former, that's not currently possible.
If the latter, you can use the Download button on the harness builder (top floating button menu) to choose an available format for an engineering drawing, diagram, or BOM or choose JSON or WireVIZ YAML; see the Export and Documentation section here: https://splice-cad.com/#/tutorial