Personally I haven't really seen built-ins in long stuff like that which is why I said likely not built-in, but it could be though! I haven't checked if that specific OP has one. I know Moitie used to make long petticoats and the OP is Moitie so it could be one of those if not offbrand. Since it does look so full compared to the stock photo (picrel, sorry the resolution is for ants) and other similar dresses, it's unlikely to just be a slip or something, but it could be a bridal petticoat! Honestly we'll never know exactly, but all of these options would work lol. Also, I only specified that there's definitely something there because you said "if any". Sorry if that wasn't clear!