I mean installing LED bulbs in the factory housing. Some vehicles (Outback not likely one of them....but I could be wrong about the newer ones) have LED headlight assemblies available for them, aftermarket or OEM, which have considerably different optics to utilize the different light source.
HIDs require a Ballast, LEDs require a driver. Frequently, the driver is built into the back of the LED bulb assembly, sometimes it's installed in the wiring a few inches from the bulb. These generate heat, so there's some form of heat sync and frequently a cooling fan on there.
The '00-04 Outbacks (I have 6...so this is usually what I'm working on) don't have very much room behind the bulb, so the LEDs with everything built into the bulb assembly don't generally fit, but some use a flexible heat sync braid which are more flexible and can fit into tighter spaces.
But basically, a drop-in LED "bulb" isn't much more complicated to install than a replacement halogen.
I recommend https://www.gtrlighting.com/ and https://www.diodedynamics.com/ for high quality replacements. Although I've used many cheap alternatives with some success.