This is what the Flux documentation says about Smart Vias:
"Overview
In traditional PCB design, designers are required to select the appropriate via type for each connection, which can be challenging and prone to errors, especially in multi-layered boards. Flux simplifies this process by automatically determining the optimal via configurations and applying them where needed.
For example, if you need to connect a trace from mid-layer 2 to the top copper layer, Flux's Smart Vias feature allows you to select the start and end layers, and it will automatically place the necessary vias and adjust the layout