The factory projector found in your classic BMW is designed for a halogen bulb. Putting an HID bulb in the projector causes excess glare and can be dangerous to other drivers at night.
These adapters sandwich between the projector lens and reflector bowl and mounts to the factory headlight housing. Other than the US Ellipsoid headlight, the adapter uses the factory mounting points and could even be removed and reverted to stock if you wanted to.
To install the new projector assembly, there is some minor modification that you will have to do to your factory headlight housings. This consists of removing some internal ridges in the housing that would otherwise prevent the adapter from fitting. This takes about 15 minutes with a dremel.
These kits include everything you need to install and run the new HID projector: HID bulbs, bi-xenon projector, ballasts, and with the Morimoto kit comes a power relay harness to send power directly to the bulbs. All components use 9005/9006 connectors and depending what is on your car now, you may need to swap over to this style connector.