It’s not what you want to hear, but I am pretty sure that as far as added flavorings go, for your bog-standard marshmallow, you’re pretty much just looking at vanilla.

Not that it’s the only thing you’re tasting, there’s sugar of course, and that’s sugar has been cooked to a certain temperature which changes some of its properties, there’s gelatine which has a bit of flavor on its own, there’s air mixed into it which affects the both feel which can change how you perceived the flavor, etc. so they’re kind of a gestalt flavor experience where the whole thing comes together as more than just the taste of whatever flavor you added to it.

Some people will say that it tastes like marsh mallow (the plant)

Traditionally they would have been made with marshmallow root, almost no one has done it that way in a long time and the marshmallows you’re buying at a grocery store almost certainly contain no marshmallow root. I’ve had some things made with the root, there’s not much flavor there, maybe a bit of earthiness, the main point of it was as sort of a thickener, which is the role filled by gelatin in modern marshmallows.