I think, irl it's not something you can ever find perfectly, because real life doesn't do perfect.
In fiction, it's the person(s) who complete and balance and understand you like no one else. You don't have to like each other, but you click. Prime example of a highly antagonistic soulmate pair is Bungou Stray Dog's Double Black, who have been described by the author as like one soul in two bodies (literal soulmates) and who from the moment they met each other clashed horribly and declared to all who would listen how much they hate each other, were on exactly the same page, a rarity for both of them to find any who understood them, let alone so completely. To the point where seven years later and after four years of separation they still have a stupid secret language.
And I think that's the thing, it's the person you need and who balances you perfectly without trying even when you don't want to them too and will always understand you even when you come back years later. It's easy and simple, even if not romantic or even platonic.
Side not, my favourite soulmate trope I've only seen once in soulmates in hatedom, who hate each other and everything else with such passion it made them soulmates in the magic soulmate deciding system. Essentially, villainous soulmates to fuck shit up together and foil each others plans. Perfect balance and understanding even in villains.
