Hi - I'd like to add fog lights to my '98 legacy - but I wanted to find out something first: How exactly do you interact with them? There is a button that turns them on/off - but then after you do that - do they just replace your high beams? Or something else? Sorry - I've never had fog lights before.

If you adding aftermarket ones you will probably wire them in so that they can be turned on whenever you want, regaurless of what other lights are on.
From the factory they would only work when your low beams are on and turn of when you switch to high beams.

Since your legacy will likely see a lot of fog & rain (given your location), imho, it's nice to be able to turn the foglights on without having to turn the low beams on. Afaik, oem fogs (on subarus) turn on only with low beams due to some legal requirement. Imho, although properly aimed low beams light up the road (and not anything too much above that like high beams), they still make enough fog glow in front of you. With a properly aimed fog (hopefully with yellow tint/hue), you can see the road better under dense fog if the low beams aren't on.
For legal considerations, you might want to replace an aftermarket fog unit's switch to a 3-way switch; one for off, one for manual (i.e. on when you push it), and one for oem default (which taps signal from the low beam). That way if you are driving in public roads, you can set it to the oem default so that there will be no legal issues. When you're driving in some mountain/hill away from public roads, the fogs can be used independently.

my 2 cents
I sometimes drive through backroad hills that can get really foggy. During those times, having the fog light turn on without low beams seem to help visibility better than having both on. Hence my suggestion.

Is the 98 legacy a ground switched system ? Would he need a relay to make aftermarket fogs work? The after market fog lights I have seen have a single power wire that is intended to go to a positive input from the low beams. Just asking?

i wired up my other car so that the fog is switched with the parking lights..so if i ever forget to turn it off, i'll go off with the lights. :)

yah, when i was out in malibu in the spring it was densly fogged...and its was much easier to drive with out the low beams but with just the fogs and parking lights on. :p