Selecting the main light from directional lights is determined by UniversalRenderPipeline.GetMainLightIndex. The priority is whether the light is sun or not, then brightness.
Currently, the main light is fixed as the first directional light in the scene lighting settings.
Lighting Window > Environment > Sun Source > Directional Light
When Sun Source is changed to None, the brighter Directional Light (1) is selected as the main light.