Will this bolt straight into my Equinox without any modifications?

Yes. It uses factory mounting points and plugs directly into existing wiring. No brackets, adapters, or modifications needed.

Does the bulb come installed, or do I need to supply my own?

The assembly ships without a bulb installed. You'll need to source the correct replacement bulb for your model year and drop it into the housing.

What's the difference between this and just replacing the bulb?

This is a complete headlight assembly. If your housing is cracked, yellowed, or the internal reflector is damaged, a new bulb won't fix it. This replaces the entire unit.