Do these headlights plug directly into the factory harness?

Yes. These units are designed with factory-style connectors and mount at the original headlight positions. No custom wiring needed.

Will the LED halos stay on when the headlights are off?

The halos function as daytime running lights, so they'll operate independently of your main headlight switch. Check your vehicle's DRL settings to control when they're active.

What happens if one headlight fails—do I have to replace both?

Each assembly is independent. You can replace one side, but these ship as a pair to ensure color match and consistency.