Wherever you summoned them. I think it shows up on the unit's display during battles. Not sure if you can find it elsewhere.
Just to be clear, the home province is where they are summoned or where the summoner is?
e.g. with a remote summon spell will their home be where they were summoned or not?
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Please, do not pretend me to draw, the one-eyed, armless, drunken monkey who is teaching me to draw says i will never be able to.